Doggie Minefields

My neighbors have adopted the rude practice of leaving small bags of dog poo around the outside of our condo. It's highly annoying. Usually they try to put the bags just underneath the bottom stair of our unit. I'm always afraid that some morning I'll walk down the stairs and just step on the bag. On top of it, sometimes it is like not all the poo makes it into the bag. So there are little poo chunks all around the stairwell at the bottom. We have adopted the practice of warning each other every time we go down the stairs, "Don't step in the poo," or "Remember, there's poo down there on the right side." I am currently considering possible retaliations for this offense. I thought I should just pick up the bag and place it in front of their door. P&AF suggested tying it to their door handle. It needs to stop!