Try the old British solution from the time of Queen Victoria. Put up a sign that says " Trespassers will be shot first, and, if practicable, questioned afterwards " ------at least , it might make someone laugh !! Also, you could put lots of pepper on the ground, like in the movie " Cool Hand Luke "------It won"t really hurt the dogs, but it would sure be amusing !! ( and the owners would be really pissed off !! ).