You could have a Hall of Losers. One that shows them in either a randomised each time or just indiscernible order so they can't even be top of the loser board.
I think either that or a big fat "BANNED/CHEATER" landing window if you click on their name. (Not that you'd be able to find them since they wouldn't appear on the high scores, you'd have to know their name.)
Adventure Quest has a system somewhat like that.