If possible, I always advocate alerting the vehicle owner of their douchness. The advisory note in the link below has been around for awhile. I usually print and keep a couple of notes at the ready. You never know when it'll come in handy. Personal confrontation is optional, but leaving one under the wiper blade should suffice.
You Park Like an ******* Helps You Share Your Feelings with Idiot Parkers
Sherwood