This is a fantastic prank, and I totally wish I could do this. It reminds me of the time that I borrowed a friend's car back in college and couldn't find a spot in her lot. There was this Ford Escape who backed in right down the middle of the line. My friend had a window crayon in her car, which washes off easily, so I wrote in big letters on the windshield, "Nice Parking, Douche-bag."
It was very satisfying. Also, just so we're clear, this is a chalk based spray paint that does not stick to the car. No cars were harmed.