What can I do to keep my neighbors dogs from crapping in my front yard?

My neighbors have two chihuahuas and they crap in my yard always. I just finished picking up 20 of their smelly craps. I'm tired of it and the smell is horrible. What can I do? Talking to the neighbors is not goint to work. My mom has, several times, tried to give them a hint about the problem by scooping up the poop and puting it on their yard, but it only worked for a while.