How to stop neighboring campers from

You've done all that you could, resolution not to your satisfaction. I've seen campers put up tape, a rope line, portable fencing or string lights to mark out what they want to claim as their territory, and this works for most folks.

I tend not to worry about this, as a campsite in a public park is not really my property, and I am not entitled to "No Trespassing" claims, which even on your own property require signage and boundary marking, and violations must ultimately be resolved in civil court.