As a dedicated bank fisherman, I know first hand how messy my fellow anglers are. I haven't fished a lake or pond where I haven't picked up someone else's trash. I have two goals when I arrive, to catch fish and to police the area for trash. Sometimes I don't catch a fish, but I always end up with a plastic bag of trash. To answer the OP's question, I never approach an offender, seems like a waste of time to me. They know the difference between right and wrong.