My own policies are clearly and humorously stated here, under sections titled Posting Comments and Trolls and Asshats — but I like Teresa Nielsen Hayden’s description of how to get your comments banned over at boingboing:
Q. What’s likely to land me in your bad graces?
A. Since you’ve asked, here’s a nowhere-near-exhaustive list:
1. Spamming. Linkwhoring. Re-posting text you’ve already posted on a dozen other sites.
2. Making supercilious and unpleasant remarks in a civil liberties thread about how the victim had it coming. This is not to say that victims never have it coming; but there’s a species of internet demi-troll that appears to specialize in posting such comments. Try not to look like you’re one of them.
3. Making snide comments and insinuations about the editors. That’s right out. You don’t like one of the editors? Take it up with them in e-mail. If you’re going to comment on an entry, talk about the entry.
4. Being nasty to no purpose. (This is the catch-all.)
5. Using unnecessarily exciting language. Making an argument is fine. Making your argument in language guaranteed to make your hearers see red? Bad idea. It practically guarantees that you’re going to have a dumb (and therefore boring) argument. And if the argument’s not going to be interesting, we don’t see the point.
6. Jeering, sneering, condescending, or one-upping when there’s been no provocation. Telling people they’re naive idiots for caring about whatever-it-is. Like the "I’m bored" pose, it’s empty attitudinizing, and it’s remarkably unpleasant.
7. Failing to notice that there are other people in the conversation. Posting a remark that’s already been made five times and answered six. Coming back and re-posting essentially the same material after a twenty-message thread has discussed your previous comment. Trying to forcibly wrench the conversation onto one of your own pet topics. Posting a stale, canned rant you’ve posted a dozen times before at other sites. Not coming back to see how others have responded to you.
Why post comments at all, unless you expect to be read? And if you expect to be read, you must know you’re part of a conversation. Therefore, you should act like it. Engage with what the other commenters are saying. Read the thread before you add to it.
8. Posting a snotty but otherwise worthless anonymous comment. It’s a lot easier to get away with snotty comments if you’re a registered user.
9. Dragging in one of those topics that’s guaranteed to generate a huge thrash that goes nowhere, like gun control, abortion, or Mac vs. PC vs. Linux. You’re only allowed to discuss those if (a.) they’re relevant to the entry; and (b.) everyone in the discussion is doing their level best to say something new.
10. This list will undoubtedly get longer.