I'm with you, man. WAAAAYYYYY to many people these days have no consideration for anybody else.
May I suggest getting a set of those little stand-up fencing units that dog owners sometimes set up to contain fido and using them to fence off a "dog run" on the back side of your lot. Don't need the dog, just the fenced area blocking access.
Alternatively, take note of where they are camped and make a point of strolling through their site while they are outside eating dinner or something. Be sure to help yourself to a hot dog or a handful of chips off somebody's plate. Might not get the message through, but ought to give them something to think about.