How about the League of Legends approach? Show that you behaved good for 6 months after your last warning and you have a 'clean sheet' again.
Removing all warnings might be a bit too much for a game like GunZ. However, -1 warning every 3 or 6 months if the player did not receive any new warnings during that time might be reasonable?
This obviously does not count if a player exceeds a certain line: DDoS, racism, and similar offenses.