Twice.
1st time: Me and someone else (Jinjo, I believe) were typing that we were peeing on each otheras a joke. He got off with a warning, I was immediately banned. Therefore, I consider this particular ban unjust and stupid, seeing as how it says nowhere that you can't make this joke and I was banned without warning.
2nd time: Spamming a link. I'll be honest, this was a legit ban.