Hotdish, I was once at a meeting during which a [vile] woman was trying to stop funding for our public library. She was the recipient of many glares. This woman had sat down near a stranger who was at the meeting for another purpose. The stranger seemed embarrassed during the meeting, and during a break made a point to tell the library supporters that she was not "with" the fund taking woman, didn't know her, and had told her during the meeting that she was in the wrong.
I suspect that PennyandPleased is concerned that people will think that she shares the views of the talker.
Sorry to veer off topic momentarily, but who in the world would want to stop funding for a public library? That just seems bizarre to me!
Continuing the off topic--Probably someone who wanted the funding to go to something else. I know a town where there was a choice between funding an expansion to the library or adding some features, like a playground and dog park, to an existing park. Both projects had merit, and the town was pretty divided on the matter.
Back on topic, I think a whispered, "Sorry, I'm trying to listen--it sounds important!" is enough. Even if the person next to you keeps talking, you don't have to pay attention or otherwise show that you care about what he's saying. Maybe he'll get the message and talk to the person on his other side. Or the rest of the audience will turn and "Ssssshhhhh!" him.