I think that you should allow a user to "purchase" warn off of another user for a set amount of forum gold. That way well behaved users can be rewarded for being at 0% since you really don't lose any rights until 30% anyway. Users would only be able to purchase 10% warn at a time (once every 30 or so days) and in order to buy it they would need an older join date than the account that they are purchasing it from (to prevent multi's from buying their own warn)