I've never been banned on a forum before, and the only warning I have ever received is on here actually, but when I looked back on the thing I got warned for, I totally agree with it and it was stupid of me for doing so.
I did once however, get suspended for a week from Habbo Hotel (when it was cool way back when), which was for starting a big argument with some dude, which lead to some unsavoury things being said on my part, and I was reported for it.

I was also on one forum where you can get instantly banned for posting the 'lolwut pear' image, or linking anything to a certain website (I don't know the rules about mentioning this website on here, so I'm not risking it)