My late uncle - a very upright, decent sort - had this problem year ago when a woman would walk her dog off the lead past his house. Well - she walked past it and the dog would go up the drive and leave some 'junk mail' on my uncle's doorstep. Having seen this happen my uncle removed aforesaid litter on a shovel and followed the woman back to her house, waited for her to enter and shut her front door before ringing her doorbell. As she opened the door to him he smiled and said 'You left this at my house, but I am returning it to you as the rightful owner' whereupon he tilted the shovel until it was empty. Nice one.