Block their door with their trash and leave a note that says “A gross slob who leaves their trash everywhere lives here” or something else to publicly shame them. That is what I used to do with my terrible gross neighbors who left trash everywhere. Bonus: you get to watch them be pissed off about it as they struggle to get out of their house.

Alternatively if they have a back yard you can throw it back in there with a note that says “you seem to have misplaced this so I returned it.”

Good luck. People are awful.