I've been banned from Youtube and 4chan for acting "too stupid."
All joking aside, I've only ever been banned from TF2 servers. One of them was for asking a pyro for help guarding my sentry while I set it up, and then the pyro started yelling at me. Turned out it was an admin who thought that engineers "aren't team players and don't need help setting up sentries, they are overpowered anyway" (quoting him as closely as possible). I disagreed with him, he threatened to ban me permanently because, again as closely as possible quoting, "I hate whiner noobs like you, I have over 200 hours on this game (note: He only had 4 as an engineer), and I know more than you about all the classes. Stop whining or I ban you." I said he probably didn't play engy much. Ban.
Another time I got banned on a nearly empty server because me and my friend were playing and I was too good. My friend was boosting the heavy acheivements with another heavy, and I was guarding the intel as an engy. After successfully getting a total of 20 kills on the admin while he was a spy, and a further 15 when the two players on the other team both went spy, he banned me and my friend.