Dealing with Obnoxious People

I hate to write a cryptic post but there's something niggling me that I'd like to write about, without naming names.
Twice now I have gone to a meeting where a woman at the meeting that I don't know has made blatant rude comments about a few people I'm connected to and I really like. It's not really my place to defend them or to get in a slanging match with this woman and of course a meeting would not be the kind of setting to act unprofessional, but this woman just really annoys me. Last time she did it I tried my best to stare intently at her in the hope that I would develop superhero powers strong enough for my gaze to burn into her thick head. No such luck. Not only did I not burn a hole in her head but she also refused to make eye contact with me, so obviously she knows that I find her comments about those people offensive. What she is saying is wrong and mean spirited. Slanderous almost. I have told the other people what she's saying and I guess I just watch her at future meetings and if she says something out of order I .... do what?
What I'd like to do is shut her up, right there and then and let everyone at the meeting know that she's lying and that she has ulterior motives for saying what she's saying. But that's not how one acts at meetings is it and it will probably make me look like a bully. But I can't sit and do nothing and let this woman keep spewing lies about these other people. Ahhh, what to do.