Even if they write a long paragraph of just fighting words as long as there is one sentence in it that is a contribution we delete everything else and keep that sentence and then write a private message to them saying “Because you violated the code of conduct we deleted these and these words. This part is great, so we thank you for this contribution.” There is an old XKCD comic that says that the YouTube commentary system would vastly improve the discussion quality if the system read aloud back what the person just wrote. This is our way of reading back aloud the trolls. They reform very quickly and we get high-quality discussion. This is a very minor technology point but I think this is the one that made the most impact.