S.L.
Have you tried just having a conversation with the guy to tell him how rude you think it is and that he should respect your property?
Most people are ignorant of the fact that they are morons. Maybe he doesn't realize how big of a deal it is. Squeaky wheel gets the grease...if you say something a couple times and he's really that big of a butt, he might walk the dog elsewhere just so he doesn't have to listen to you complain.
If I were you, and it happened again, I would pick up the poop and place it in his front yard or even on his front porch, with a note stuck to it asking him to please be more considerate. In big bold black permanent marker so he can't miss it...fight fire with fire! Sometimes diplomacy is overrated.