By breaking the rules, they are in the Chit-Chat Forum pinned I believe. 3 warnings = banned, no coming back either. EDIT: Im sure a mod or admin would tell you how many warnings you have.
Each member starts off with 3 "chances" (I guess) to waste. If they recieve 3 warnings then its a ban, no questions asked. However, if it is a very serious incident, then an instant ban may be applied. You can be warned in quite a few ways, I can't be bothered to type all the ways, so you can check by reading the forum rules. You can tell if you get a warning by a mod/admin/memberofstaff actually stating that you have a warning.