I'd call the city and see what the laws are. Here there is a leash law, as well as a poop law. You cannot let your animal defecate and not clean it up, a person can get fined, and after so many fines won't be issued a renewal license for the pet, and the pet can be taken away by animal control.

That's just so far inconsiderate of your neighbors!! Please don't take any drastic measures on your own, that will only lead to their retaliation and possibly trouble for you. Take the high road here and do things by the books, these people need to learn about the responsibility of raising a pet.

hugs, Lisa