I usually bend their wing mirrors in the direction they're least keen to go in and then pull up all the wipers. I doubt it gets through their thick skulls, but the minor inconvenience might at least reflect the more significant inconvenience they're inflicting on anyone trying to get by their thoughtlessly parked car. If there's any litter, I may take the opportunity to construct a little shrine to their stupidity on their bonnet.
Of course, as mentioned, they leave their car there long enough some passing drunk will be a bit more belligerent with their displeasure. Never mind.