Everyone starts with 0% (out of 100%).

For each offense, there's a certain amount of percentages added to a user's warn level.

For example, for spamming, it could add 10% to the user's warn level.

Repeated offenses (like viciously hara.ssing someone else, or bashing on a fan club, or just straight out insulting someone else, more spamming, etc) would add to the warn level. 

Of course, each offense would have a different amount of percentage, doesn't necessarily have to be 10% or anything. 

Once it reaches 100%, they are banned from posting in the forums forever (or a set amount of time).

Don't know if anyone else has suggested this, so I'm sorry. ;_;