Do they have the same landlord as you? If so, can you bring it up with the landlord? Or is there maybe a neighborhood association you can talk to? Chance are you're not the only one who is upset about this issue.
I would be pissed about this. I hate when people don't clean up after their dogs.
Are you sure it's this person's dog? Have you seen the dog go and the guy not pick it up? If so, tell him he needs to start picking up after his dog when it goes in your yard. If he doesn't, then call animal control and see what they can do about issuing a ticket.
I know, but you can't say anything until you see it happen with your own eyes. Annoying, I know. Maybe when you do see them walking, you can wait by the window with a bag ready. When they go to leave your property, open the door and call out, I have a bag here please pick up after your dog.
you could but a sign on your yard? but other then that, not much you can do really. and it gross. My neighbor lets there dog poop on the sidewalk outside our door (we share the same walk way) and eww it say there for days.. for daaaays! and their dog and they neighbors dog have sex outfront. ewww for all the neighbors to see.
If you are sure it's their dog, I would collect the poop on your property, put it in a brown bag, and leave it on their doorstep with a note: "I think this is yours, wanted to make sure to return it."