The situation is frustrating, because no matter what you do, you're the bad guy. I've little experience myself in the public forum when dealing with nasties, but generally I've found being nice doesn't work too often--they take it as a sign of weakness.
Try to answer every "comment" or "look" they have with a question. Try to say as little as possible, and don't explain yourself. I always try to throw it back in their court--short responses that leave THEM having to explain themselves. Painful silences that make THEM squirm, then move on to the next topic. My favorites for nasty, personal comments are the most direct approach:
1."Why would you say that?"
2. "What was the purpose of that comment?"
3. "Have I said something that angered or offended you?"
4. "Is there a problem?"
These have to be asked with the sincerest look (you can muster) of being genuinely interested in what they have to say so you don't come off as being the aggressor. It's difficult for them to answer 1 or 2 without looking like a COMPLETE ass****, so that generally shuts them up at least. With 3 and 4, if there is a comeback, usually these responses work: "OH--I must be mistaken--I thought by the tone of your voice (look on your face)that you were unhappy with something that was said," or "Yes, I could tell by the tone of your voice/look on your face you weren't happy with _____," then you just move on.
I have, in moments of immature weakness, also been known to say, "Your powers are no good here--be gone! Before someone drops a house on you too!" and "You're so good at being (a/an)_______. Do you have to work really hard at it, or does it come naturally to you?
I've found that people afraid of confrontation, even when they're in the right, are the ones who tend to be victimized by these negative types the most. Even groups can be bullied, because people are afraid of rejection, even if it is by someone they can't stand. It's been so ingrained in us to worry about other people's feelings that we sometimes forget that OUR feelings count too!
Good Luck!