Just go take a shit on his doorstep every time dog shits in your yard. Be careful about harming someone's pet. I know if their like me it would be detrimental to not only the one who harmed, but maybe his whole family. I like my dogs way more than most people I encounter. At the same time I want allow my dogs to cause problems either. It's an owner problem because he don't respect other people. Talk nicely with him and explain the problems your having and if it doesn't change the situation punish the owner!