iChat - Select the name, hit "delete" (or backspace, whatever it's called)
Adium 1.6 and lower - Delete the name from the contact list manager
Any Adium alpha - Delete from contact list manager, and theoretically, this change should also affect iChat, since they use the same chat agent
If you've done all that, I can't think of what else would be wrong... basically, Adium 1.6 is the only thing that uses its own contact list (I can't speak for Trillian though, having never used it).