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By probably anyone, really. As I started voting in Tribunal I realised this during reading one of the logs "I have been reported a zillion times", "(Player said they reported him. His reaction:) Guest: Yea, but I just get a week ban for this".
Really, those who want to be rude simply don't care if they get ban on one account for a week. They just create another account and go as they want. Some of them don't even start a game, only bring chaos and are rude to others.
The ban system here is really lacking in an effectiveness.