Thanks for writing. I'm sorry to hear about your frustrating situation!
There are a number of sprays available, but I've heard mixed reviews on how they work. Some gardeners love them; others felt they were a waste of money. So they may be worth a try if you keep that in mind.
Is it possible to put up a small, decorative fence along the property line to discourage their dog from crossing over? That's often the easiest solution, especially if they don't have a large dog.
Other than that, I'm afraid I don't have a good solution other than talking to your neighbors and convincing them that their dog should not be making deposits on your property. After all, dog poo can transmit diseases to humans --- so it is a bit of a health hazard, especially if you don't own a dog yourself.
Senior Garden Editor, BHG.com
Better Homes and Gardens