It’s Called a Spite Dump
What is it with people not picking up after their dogs? It’s really not that hard. Just use a bag to scoop the poop, then flip the bag outside-in before tossing it in the garbage. If you didn’t bring a bag on your walk, tough luck. Use your hands or shirt or something.
Heed this person’s warning. They know who’s been allowing their pet dog to take a crap on his lawn and promise to get them back. Pooping on someone’s porch every week may be a bit too extreme, but drastic times call for drastic measures. Hopefully, the OP didn’t have to make good on their threat.