In today's mentality with the holy-screechie-crowd seeing SEX in rock formations and clamoring to have them blown up (I'm serious!), soon smacking someone on the butt - even between friends - will be legally construed as an illegal lewd and lascivious act. Right up there with **** and child molestation. Butt-slappers will be doing 10 years in prison if the whack-job crew gets it's way. Just watch.
Personally I'd settle for dragging them from their cars and re-arranging their bridge-work.