Mainly I've had people walk through or even cycle through my site. (I avoid the places that have sites so small that the other concerns here are much of an issue.) I've found that clothes lines work wonders for helping define the site boundaries.
One place we stayed a tent camper that ended up on a VERY narrow site next to our decent-sized site had mistakenly setup their tent on the front part of our site before we got there. We just left them alone, and they left us alone - it was pretty clear why they had done this and that their site was only workable for a self-contained RV. They also liked playing with our dogs.
sdianel wrote:
you can't fix stupid.
The fix: Education.