We had great success just putting up a little sign on the door that said “no unaddressed mail”.
Of course since we were buying a custom sign for it we also added “…or else”, and one time I just happened to open the door after a young lad had pushed a leaflet through. He sort of stepped backwards a bit and his eyes went down to the sign and he stuttered “I’m so sorry I didn’t see the sign” as if he thought I was about to wallop him.