I'm surprised you can be on LJ long and not realise that just about anyone can stumble across your posts - I can't have been here more than a couple of months when someone random turned up in my journal comments because they had a google alert on the topic. Obviously that's a little different from a fandom dogpile and, since it's a space I have some control over, I expect visitors to realise that its within my rights to shut conversation down if I think its getting out of hand - that's not ever happened but I got close once to telling two folk they were getting out of line and to quit laying into each other.

I've never felt the need to post "rules of engagement" but then I'm not a big fandom fish. If I were to feel conversations were getting out of hand a lot on my LJ then I might do something like that in my profile, just to make it clear that while anyone was welcome to come and talk about a public post, this is my discussion space and I expect that discussion to happen according to my rules and that means people being polite to each other. Just like you don't trash someone's house simply because they've invited you inside.

So, I guess, I'm saying there seems to be an area where you're happy for people to visit and comment but it isn't exactly a public arena for no holds barred debate. Then of course you have to worry about where the line between "polite discourse" and "censoring anyone who disagrees with me" falls.