Recalcitrant parkers at my place have returned to their car to find a 3 x 5 inch sticker stuck to the drivers side window with my hand written note advising them of their errant ways. The 10 minutes it takes them to remove it usually gets them thinking about future breaches. If you do something similar you must not make it dangerous for them to drive away with impaired visibility, so avoid areas like the windscreen or side mirrors.