First of all, and most important: If I sense an argument is getting out of hand and devolving into wank, I will tell the people involved to stop. And when the mod says to stop, it means you stop. NOW. Take it to private message or email or AIM, just keep it out of the community. If I can plainly see people aren't listening when I tell them to stop, I'll freeze the thread. No exceptions.
Second: There are different definitions of bashing. Bashing doesn't have to be rabid stupidity and ALLCAPS MCSPELLSBAD, it can also be an otherwise coherent diatribate on why suchandsuch character is a moron who doesn't deserve their love interest/needs to be kicked in the face by their mentor/is a Mary Sue/Gary Stu simply because that fan doesn't like them. And if it comes in a reply to a defense post about the character then yes, it is bashing. You don't HAVE to agree or like the character, but you know that lame little Mommy rule "if you don't have anything nice to say, don't say anything at all"? It applies here. Big time. People put a lot of thought into their essays, and it's rude to come barging in with "I just think s/he's a big fat Author's Pet and that's why I can't stand him/her".
(But it is still okay to mention you didn't think the writing was the best, the character deserved more development, etc. Just don't be a dick about it and if someone disagrees with you, let them. :P)
Third: If you're called out for either of the above or some other indiscretion, replying with "sorry, I thought opinions and discussions were allowed here, guess I was wrong" or "wow, I didn't realize this was SUCH a hot-button issue that'd cause people to get all up in arms about it" is just going to make it worse. It looks wanky and makes you sound like you're crying OPPRESSHUN!!! over being asked to behave yourself. Just take it like a grownup, apologize and get on with your life.
Conversely, if someone DOES pull out the above "oppression" cards, don't troll them. You're only making the problem worse, and they just might TAKE your advice and crai moar.
And as a final note, I've appointed
rezo_eris as my co-mod. :) Be kind to him! (Edited because I suck at guessing gender on the internet!)