How do I poop proof my yard from neighborhood dogs?

We are buying a house that has been vacant for a while, the neighborhood seems nice; accept it seems that the neighbors have ignored their dogs "accidents" when they have walked their dogs infront of the vacant house. Now that the dogs have made this their "spot", now do I now deter them from making in my yard, but still making the yard a safe place for my dogs to play in? I don't want to put up a fence.