A small bb that goes in a bb gun, although any small rock will do. Think the sort of thing from “A Christmas Story”.

The idea is to slowly let air out of the tire without damaging it. When they put air back in the tire(or call for help, because bad parkers don’t know how to do anything for themselves), they’ll realize that someone let the air out on purpose and maybe think twice about parking bad again.

oh and btw, the last time I suggest this on reddit, I got downvoted to hell, but jokes about physical attacks on people were being praised.