A principal I once worked for put stars on each table. If a table got loud, she removed one of the stars. I don't remember if there was a consequence/reward for losing/keeping stars, but it seemed to help a little.
I agree with Fiddle4Fun, though. When I was in elementary school, our principal tried everything to get us to be quiet. Several times he made us eat in silence. He even installed one of those traffic light things, which we just yelled at to make it turn red.
I never understood why they wanted us to be so quiet during our break. I know it's not enjoyable for the adults, but chances are, the kids are just talking, and it's loud with so many kids in there.