Kerberos saves your principals for you if you select "Remember this principal" in the Get Ticket window. Saved principals then auto-complete when you start to type them in the Get Ticket and Change Password windows.
If you have saved a principal that you will no longer use, and you do not want that principal to auto-complete, clear your principal history.
Note: When you clear your principal history, MIT Kerberos clears all of your saved principals. It is not possible to remove some principals while keeping others.