In our room we have "chaperones": parent volunteers who help with keeping the kids engaged and participating in the program - we do this because we do not require parents to stay. This is how I get parents invovled is to volunteer a month at a time. Anyway, that is part of their responsibility, and they are catching on - when a kid begins to act up, one of the chaperones will walk over and plant themselves nearby.
If I can't get a chaperone to stop them, then I will point it out from the stage - again, without the parents in the room, I almost have to some times. I think the embarassment alone has a huge effect - and I don't try to embarass them, I do it in a calm manner - but all eyes are on them and they get the point.