As long as they allow unrestrained drunks on board there isn't much a pilot in a small plane can do if a passenger acts up. Short of fitting each passenger with a pain collar, buxom drill thralls with big hair and wearing silver outfits are extra, I'm not sure there is any easy answer.
A blast from the past, A nice picture of those pain collars. And, entirely incidentally, Angelique Pettyjohn as drill thrall Shahna, who made quite an impression in my youth:
http://en.wikipedia.org/wiki/The_Gamesters_of_TriskelionI suspect that the practical answer is empowering the pilots to decline inebriated passengers without the exclusion being considered a violation of any agreement or contract.
The only other alternative to come to mind is that the dunks might be hog tied, or stuffed in a sack, and carried as cargo. Perhaps lashed to one of the floats.