That's a tough one. When I owned a home I used to get so angry at people who would bring their dogs by and let them eliminate in my yard, and let them urinate on my shrubs and kill them. I put up some short fencing around the shrubs to protect them and they went around the fencing to urinate on them! I guess I would recommend putting up some inexpensive fencing as a barrier and perhaps a sign requesting people "curb" their dogs, although personally I have also tried signs and some people just disregard those as well. Let's face it - there are just some very inconsiderate people in the world!
I suppose you could set up a surveillance system and try to catch the perpetrators on video.
You could also set up a pee post away from your property or move the excrement to a place somewhere near your property so as to entice the dogs to eliminate in a different area.
You could also make up a flyer requesting dog owners in your neighborhood to please "curb" their dogs and not let their dogs eliminate on your property - to pick up after their dogs...but if people are that inconsiderate, I doubt that would work.