My suggestion is not to scratch/dent/poke the radiator. Dont do any physical damage. Just do something that can reversible but embarrassing for the car driver.

Suggestion.
1) Keep a 1L bottle of normal water in your car.
2) Print a few pages of A4 sized paper that says "NO PARKING" or "Learn how to park" in big fonts.
3) Put the paper all over his car especially over the windscreen and driver side window.
4) Pour the water over the paper and let it dry.

The guy/girl surely have hard time when the paper dry. Need to use soap to take out the paper. But at least he wont have a dented/wrecked card and dont have to spend money to repair the car, just very embarrassing.

This post has been edited by bernardryan: Jan 12 2011, 08:54 AM