We had the same type of problem when we moved into our house 9yr ago. Our home had been vacant almost a year when we bought it & since nobody lived there our neighbors taught their Lab to use our yard instead of theirs. My husband didn't take it well, their dog was always in our yard. Since then the dog was hit by a car(in front of their house & son), they have an electric fence and dont use it as it's cruel. They have another Lab now & that dog is better about using their yard or the farmers field behind our houses. But I still find poop under my wash line & I alway scoop it up & redeposit it in their yard.
Desiree