Put the poop in a brown paper bag and put it on their porch, light the bag on fire and knock, then run like crazy!! Hahaha jk
I dont have any advice for you, sorry. If it were me I would just ask them not to allow their dog to do that. Or do what my DH would do pick the poop up and throw it over in their yard!
We live in a condo and downstaris, well our up stairs neighbor would just sweep the poo off and it would fall on our patio! Well DH isnt one to mess with and he started to put the poop in a little bag and throw it back up there lol.. they stopped in about a couple days!