I believe that a postmistress somewhere in an English village has begun writing the names of the children who buy crisps and drinks from her on the packaging. This allows her to identify any litter and to ban the miscreant youngster responsible from her shop in future.
For sellers who don’t know their customers personally, the problem of litter usually goes unaddressed.
Today’s invention is a vending machine which takes only notes as payment. Each item on sale is printed with a machine-readable code. People who buy from the machine, only receive their change if they place the ’empty’ in the tray provided. If they choose to throw their litter around, other people can get their change.
The machines might be networked together so that it would be possible to dump an empty can in a different machine to the one from which it was bought and still get the correct change.