We once were in a site that was close to the showers and we had dozens of people cutting through our site between our picnic table and awning so they wouldn't have to walk an additional 200 feet to stay on the road and go around us and the site beside us. At first I didn't say anything, but I placed the grill, chairs and other objects across the cut-through area to coerce people to go around us. This didn't work -- most people just snaked their way through the obstacles and some actually moved my stuff out of their way -- this, with me sitting right there for them to see. I began to get really mad at this blatant disregard for my privacy and "property" so I decided that I would just make each and every "trespasser" aware of what they were doing and ask them to please stop. Most people were not very receptive, ignoring me and carrying on their way. Some people apologized and thanked me for the lesson in campground etiquette. Many of those who were not receptive at first tried walking through again, on their way back, but none of them made it all of the way through because I made it very clear to them that it was not in their best interest to proceed.
Yes, I am sure I was tagged as the campground curmudgeon, but I didn't care because by the end of our stay there, nobody was cutting through.
I now pay close attention of what is around my site before I make a reservation.
Bruce