Scrump; the best way of dealing with natterers is to play quieter, until it becomes obvious too them and others that they are talking too loud. Then go back to 'normal' volume. You can extend this a bit by quietening the rest of the audience 'so we can all hear this fascinating conversation'. The odd quiet aside can be good as well; if you can get the rest of the audience to laugh at the problem ones, that usually shuts them up. Having said that, sometimes drastic measures are necessary; I remember a young woman standing between our duo and the audience, yelling into her mobile phone (we were playing 'unplugged' to an audience of about 50.) We moved out from the stage, stood either side of her and played as loud as we could into both ears. She moved away, (still talking) but we followed her, as far as the ladies toilets. Lots of applause, and I bet she didn't do it again!
Tim from Bit on the Side