OK, nobody liked my suggestion of a refundable deposit fee on the WAG bags, so how about this (originally suggested by Daves Not Here, not sure if he was kidding)

Print a unique serial number on every WAG bag. Write down everyone's WAG bag number when they pick up their permit. Then if a ranger finds abandoned WAG bags on the trail, he can look up whose bag it was, and send them a ticket for a $1000 fine.