Boddie, I come accross this quite often. I was going up to feed the foxes the other night when I was just about to pass this car when I saw all sorts of rubbish come flying out of this car's window so I stopped behind the car which was about to leave blocking them in, I got out of the car and told them I wasn't going to let them out until they had picked it all up, after a lot of abuse from them I told them I was an off duty policewoman and they would be in trouble if they didn't pick it up and guess what they picked it up and appologised and said it wouldn't happen again
they were also parked about 3ft away from a rubbish bin. I often pretend to be a council official or an off duty policewoman and it always works
You should try it sometime