AGGS ISLAND, England

AS Steve Linford walks his German shepherd, Zen, across the gangway from his houseboat into his prim little garden on this small island in the Thames, he hardly looks like a man in a battle over the future of cyberspace. He has a salt-and-pepper beard and a twinkle in his blue eyes, but the effect is more former hippie than Sean Connery.

After Zen gives a good bark at the ducks, the two return to the boat, and Mr. Linford climbs a spiral staircase into a sunny home office with nine computer screens piled on a black desk. This is the unlikely command center for the Spamhaus Project, one of the leading groups that is trying to make the world safe from junk e-mail.

As a cause, stopping spam may not be as urgent as, say, curing AIDS. Yet thousands of activists, of whom Mr. Linford may be the most visible, have mobilized to fight it.

By some counts, spam is now as much as 80 percent of all e-mail. It is a drag on human endeavor, in the sense that people collectively spend billions of seconds each day opening, puzzling over, complaining about and deleting messages from charlatans and pornographers - and, yes, legitimate if unloved marketers.

"E-mail is the most incredible communication vehicle invented, and it is on the verge of being made useless,'' Mr. Linford said.

On the floor of Mr. Linford's houseboat office, near the Hampton Court palace of Henry VIII, just south of London, is a cube-shaped Apple computer that is the nerve center of Spamhaus, controlling servers on five continents. In its database are dossiers on the 200 most prolific spammers and the addresses of the 8,000 computers they use to inundate people with ads. Spamhaus makes the list available to Internet service providers, which use the information to weed spam from the e-mail boxes of 160 million users.

Those lists are compiled by Mr. Linford and 15 volunteers, many of whom work for Internet service providers. Some members of the group do detective work, tracking down the spammers from telltale clues they leave in their e-mail. Others assemble this evidence to try to persuade the service providers to kick spammers off their networks. One Spamhaus member, a Southern California woman who goes by the online name "Shiksaa,'' chats online with the spammers, pumping them for information and trying to pull them away from the dark side.

"They are kind of like the X-men,'' said Matt Sergeant, director of anti-spam technology at MessageLabs, an e-mail security firm in Britain that works with Spamhaus. "Each one has their specialist powers.''

Spamhaus plays up the comic-book theme a bit. The main screen of its internal computer network is emblazoned with Spider-Man's slogan: "With great power comes great responsibility.''

Not everyone sees Mr. Linford as a hero. Most of the marketers that are his targets say they don't send spam; they call Mr. Linford a vigilante. And the Internet companies he pressures to stop doing business with spammers say he sometimes pushes too hard. He is known to have blocked the e-mail of Internet service executives he thinks aren't kicking off spammers fast enough - a method that often wins results, if not friends.

Yet Mr. Linford, 46, has earned the respect of most Internet service providers - even those with whom he has had run-ins - as the best source of information about spammers.

"Spamhaus is the only clearinghouse for information on the spammers themselves, and for that it is invaluable," said Laura Atkins, who runs Word to the Wise, an e-mail consulting firm in San Carlos, Calif. "Any time one of my clients has ended up on their list it is because someone received mail they didn't ask for."

Mr. Linford has focused on making his list of spammers reliable enough for big companies to trust. He publishes his e-mail address and phone number and responds to complaints that listings are incorrect.

That is in sharp contrast to other spam-blocking lists, which are often run anonymously and, at times, recklessly. Some block the mail of innocent Internet users to create pressure on the Internet provider to kick off spammers.

For now, sending unsolicited e-mail isn't illegal in the United States, but it has just been prohibited by the European Union. Most Internet providers have policies that ban spam from their networks; some providers have sued spammers, contending that tactics used to avoid detection are illegal.

Mr. Linford says he has intercepted chat-room conversations between spammers and crackers, the name for malicious hackers who write computer viruses and steal credit card numbers. The spammers have been seeking ways to send their messages to avoid the blocking systems created by Internet providers.

"In the last six months, the cracker world has joined the spammer world,'' Mr. Linford said.

Aided by crackers, the spammers have secretly infected and taken control of thousands of computers around the world, most of them owned by home users with high-speed Internet connections.

These machines - called zombie drones - relay mail for spammers and serve as hosts for the Web sites where people are sent by spam, all without the computer owner's knowledge.

Since last June, zombie drones have also been subjecting Spamhaus to what is called a distributed denial-of-service attack, perhaps the most virulent weapon in a hacker's arsenal.

Tens of thousands of enemy machines have simultaneously deluged Spamhaus's computers with so much meaningless data that they can barely perform their intended missions. Similar attacks have put several smaller anti-spam organizations out of business.

This month, the crackers took the attack to a new level: they released two computer viruses that have already spread to hundreds of thousands of machines. The purpose was to attack Spamhaus and two similar groups.

"For the spammers to actually manufacture and release a worldwide virus specifically to attack you, you're probably making quite some impact on them," Mr. Linford said.

HOW did Mr. Linford end up as an avenging angel of cyberspace?

Discouraged by the economic stagnation of England in the 1950's, Mr. Linford's parents moved to Rome, where his father ran a factory that made industrial platinum. Steve Linford dropped out of a college photography program, bought a motor home, parked it on beaches and played his guitar in coffee shops for money. He eventually met Ennio Morricone, the legendary Italian film composer. (Mr. Linford can be heard singing on the soundtrack for "Copkiller," a 1983 Italian film starring Harvey Keitel.)

Mr. Linford later became a road manager for acts like Pink Floyd and Michael Jackson when they toured Italy. As he saw technology embrace music production, Mr. Linford became enamored with computers. In 1986, he drove the motor home back to London and started a company devoted to putting musical tours online. It flopped, but he did start a Web page design and hosting business, called Ultradesign Internet. It was there that he had his first run-ins with spam.

Mr. Linford's initial reaction to spam was similar to that of countless others. Outraged, he asked the senders to remove him - and his clients - from their lists. Getting no response, he turned to the Internet providers. After he failed to get results there, an activist was born.

Mr. Linford found the central meeting place for the anti-spam activists - an Internet newsgroup that is called Nanae, for news.admin

.net-abuse.email. Like many other news groups, Nanae (pronounced nah-NAY) is a boisterous place, where information about fighting spam is interposed with rather pointed insults of spammers and their allies.

"Nanae is a very angry crowd," Mr. Linford said. "They shout a lot because they feel powerless."

Mr. Linford, however, felt anything but powerless. In 1997, he created a series of sophisticated Web sites with tools to help spam fighters, databases of people selling software for use in sending spam, and assistance for people who wanted to write to an Internet service provider to complain about spam.

Because he owned an Internet company, Mr. Linford encouraged activists to use far more moderate language, without the typical threats and demands. In 1998, he started what would become his main site: Spamhaus.org, a clearinghouse for information on the organizations behind most of the spam. Meanwhile, Paul Vixie, the pioneering Internet software developer in Redwood City, Calif., had formed the Mail Abuse Prevention Service, creator of the Realtime Blackhole List. That was the first list to block Internet addresses known as sources of spam. But that effort became bogged down, both by lawsuits and internal bickering.

So Mr. Linford created his own list, the Spamhaus Block List, devoted to addresses used by spammers. He says it is used by Internet providers that serve 160 million e-mail users.

That count is impossible to verify. In the United States, the list is not used by the biggest providers, like America Online and Microsoft's Hotmail. But it is used by the next tier of providers, including the Road Runner high-speed service, from Time Warner, and the NetZero and Juno services, from United Online. Smaller organizations that cannot afford commercial anti-spam services also depend on the list.

Spamhaus takes no money for its services, and the computers it uses to host the service are donated. So far, Mr. Linford has paid all of the direct costs, about $25,000 a year, using money from Ultradesign, the company he still owns and runs.

That will have to change, he acknowledged. He and some of his volunteers have outstanding legal bills from defending a lawsuit, now dismissed, brought by a group of Florida e-mail marketers. He has asked the British government for a grant, but has not received one. Whatever the source of funds, he says he hopes that access to his services will remain free.

JUST as people talk about their ailments when they meet doctors, people can't wait to show Steve Linford their spam.

A visitor shows him one for "superviagra.'' Mr. Linford ignores the return address, which the spammer made up. But he looks closely at the address of the Web site being advertised. Fingers flying, he looks up the site in the "whois," the database that links domain names to Internet protocol numbers, the unique address of each computer on the Internet.

Mr. Linford then looks up the number in Spamhaus's block list.

Pay dirt. The site is operated by Chinanet Chongqing, one of the regional state-owned Chinese Internet providers. According to the block list, it is operated on behalf of Alan Ralsky, an e-mail marketer in Bloomfield Hills, Mich., whom Spamhaus calls the world's No. 1 spammer.

Mr. Linford looks at a second e-mail message, this one for mortgages. The same drill leads to an Internet site hosted from Brazil. But again, the block list already knows about it.

"Ah, Ralsky again,'' Mr. Linford said.

On its Register of Known Spam Operations, or Rokso, Spamhaus describes Mr. Ralsky as "one of the bigger spam houses on the Internet with a gang of fellow morally challenged types working with him." The files include state records of Ralsky's run-ins with the law, newspaper articles about him, and long lists of aliases and Web sites he supposedly has used.

Mr. Ralsky, in a telephone interview last week, said of Mr. Linford: "He is so far off base on me he has no clue.'' Mr. Ralsky said he sold travel and other products but did not handle "super Viagra.''

I don't see where he's coming from,'' Mr. Ralsky added. "All we are doing is selling products. I don't understand why I don't have the right to make a living.''

Mr. Linford, of course, disputes this, saying that his investigators have traced the Internet domains used in these spam messages to companies controlled by Mr. Ralsky.

At its peak last year, the Spamhaus Block List was catching as much as half of the spam at many of the providers that used it, according to Mr. Linford and Internet services. But its effectiveness has fallen sharply as spammers use zombie drones and other techniques to hide their tracks. Many Internet providers now say the list is catching less than 10 percent of the spam. But there are other block lists, focusing on identifying purloined computers, that are now much more effective in keeping spam out of inboxes.

Eliminating these zombie drones has become a major headache for providers of high-speed Internet service. They must call users, explain that their computers have been secretly invaded and talk them through the extensive steps required to remove the problem.

Even though Mr. Linford's block list is faltering, his database of spammers remains a potent force in the fight against spam. Many Internet service providers still check regularly with Rokso to vet potential customers before they open accounts. And they monitor the block list to see which spammers have appeared on their networks.

Mr. Linford said some of the big American service providers, like Qwest and Sprint, now respond quickly to cut off spammers named by Spamhaus. Others take more persuading. For example, Spamhaus complained for months to Cogent Communications that Eddy Marin, one of the perennial top spammers in Rokso, was using its network. Cogent finally cut off Mr. Marin's account late last month. A spokesman for Mr. Marin said Cogent's action was not justified.

As for Cogent, Michael Hammons, the company's senior director for operations, said Spamhaus often pressed it to cut off customers based on what he saw as flimsy evidence.

"I'm concerned that we should find customers guilty by association or alleged association," he said. "They may give us a warning to say you will have problems with this customer, but we can't do anything until we actually do have problems."

Mr. Hammons added that Cogent found the information from Spamhaus to be more credible than that from any other anti-spam group.

NOW, Spamhaus is trying to win over Internet providers around the world, especially in China, which has become the headquarters of choice for many spammers. Spamhaus has blocked the corporate e-mail of Chinanet-Shanghai, one of several state-owned Internet providers. In response, the company created a department to look for spammers.

"We don't like to see that we are blacklisted," wrote Lin Chen, an administrator at Chinanet-Shanghai, in an e-mail interview. He called the blocking actions of anti-spam groups "functional and effective." Spamhaus, he wrote, promptly removed the block when the spammers were cut off.

Despite efforts by Spamhaus and others, the volume of spam appears to be increasing. Spamhaus's campaign is futile, said Scott Richter, president of OptinRealBig, an e-mail marketing company in Westminster, Colo., which is on the Rokso list.

"All they are doing is making the problem 10 times worse,'' said Mr. Richter, who says he sends e-mail messages that are requested, not spam. "The spammers are learning to do stuff that can't be caught. If they get kicked off a Chinese I.S.P. they open the next day at a Korean one, who never had a way to get that sort of customer before.''

Mr. Linford said he believed that spammers could be contained, if not eliminated. A tough new anti-spam law in Europe will help, he said. The proposed Can-Spam act in the United States, he said, is not tough enough, but he figures that when it fails to work, Congress will have to make a stronger law. But Mr. Linford gloomily predicts that spammers will simply move more of their operations to Asia and Latin America.

As for Mr. Linford, he plans to move his home, business and Spamhaus to a 70-foot yacht that will travel, cove to cove, across the Adriatic.

But spammers had better not relax. With superfast satellite connections, he plans to hunt them down from the high seas.