Maybe just be more direct and authoritative. “Sir! NO ONE LIKES YOUR RUBBISH! PICK IT UP!”

I yelled this at a man who tossed his litter on the gardens of the beautiful campus of King’s College London. He was too startled to respond, and he attracted the condescending gazes of other pedestrians. He immediately picked it up, no questions.

I’m not usually a fan of public shaming, but I deeply care about the environment.