If they come in after you, make it look like they camped in the area all of you take care of business in...
When the chance comes, put a roll of toilet paper on a stick close, and half buried TP sticking out of the ground.
Then just watch.
Did this on an elk hunt where people were parking in our camp with about 100 yards of good parking before our camp.
It was hilarious to hear the driver and passenger go back and forth about parking.