I was in Asda car park a couple months back, and somebody had parked his car with the passenger side right up to the door of the car next to him - the cars owner (old enough to have difficulty getting in the other side and climbing across), had been stood there 45 minutes waiting for this other inconsiderate idiot to come back.
As we drove into the car park, he was getting right up in this woman's face, shouting and swearing - I dunno what she said to him but whatever it was, it was well justified. If I saw anybody talking to my mum like that, they'd get their lights punched out and no questions asked. I tapped him on the shoulder and less than politely told him to get in his car and go away, just as store security arrived.
In any case, I wouldn't have done anything to his car as he would have just left it there longer, plus you're the wrong side of the law if you do. I'm 6ft3 and 23 stone so I don't often get any backchat - it comes in very handy.