I've had 2 cars keyed (including my skyline while on display at the GP), but that was by coward f**ks that were to scared to say something to me, so instead they attack your possessions.
Bit different in rim specs case, if they hit it hard enough to dent it and take paint, then there's no way they wouldn't have known they'd done it.
My current car is a piece of shit, I close the door with decent kicks in steel toed work boots, but I still take care in car parks for the sake of the car next to me, no matter how nice/shit it is. Occasionally I've knicked a car next to me, but never left any damage, it takes a decent hit to do that.
And like rimona said, they'll no doubt be thinking about how their car was keyed as a result of their carelessness, so in future they'd no doubt be more careful. Who knows jeppe, they might park next to you one day, maybe you should thank rimness.
And there's a word for how u just forgive everyone that wrongs you, it's 'pushover'