[EricH]

I actually think that--should it actually be necessary--this one is enforceable, and has some teeth.

Enforceable: If it's noticed that there are paper airplanes (etc.) in the air, someone notes where they're coming from. It is then announced over the PA that "Will the team(s) in Section XYZ please stop throwing paper airplanes and clean up the mess" down here. (Team numbers used if they can be determined from long range.) Send a couple crowd-control volunteers (or venue staff) up there.

The Teeth: If a second announcement is needed, invoke the Civility Rule (last year's T6). After that... well, I can't say that I've ever seen that rule be enforced past the discussion stage, because usually the discussion clears it up.

That might be a bit drastic, but it should work...