This is a tough one. It would be best if the park ranger could deal with it. I would avoid confrontation - you never know how crazy people are. Remember - "Never argue with an idiot - the will bring you down to their level, and beat you with their experience.". We camp with a dog. I think I would let her do her business throughout the weekend along the side where they are coming through and not clean it up until we are ready to leave. We would never normally do that - but that might discourage the use of our campsite as a highway.