Why do dogs keep crapping on my lawn?

We have like a ten ft. square lawn and some dog, or dogs, keeps crapping on it. Once I could forgive. I have, myself, been caught without a plastic bag on a few occasions. But now it is starting to look like someone is training their dog to poop on our lawn. I mean come on people! What is the best way to deal with this situation? I have considered sitting on my porch with a shotgun twenty-four hours a day...but I have no shotgun and it is getting cold. I could make a sign, which the dog owner would promptly ignore. Plant cacti in my lawn? I am at a loss. There is certain dog etiquette that needs to followed in a civil society. What is the world coming to when shit can be left anywhere? In the privacy of my own backyard, fine, I will leave crap festering for a few days, but on the front lawn, by my walk? That is a slap in the face. Suggestions anyone? And by the way what the heck is "curbing your dog"? I assume it means pick up their crap, but where did the phrase come from?