Hi Mate
I feel for u, had a similar problem a while back when i had a black Rx-8 kids loved to play around the cars which pi**ed me off greatly then one fell off their bike with their handle bar scrapping down the side of the wing. i was gutted to say the least, My ex-wife had a black FTO and she got numerous dings on her passenger door, i think caused by kids swinging open their parents car door. I hate public parking and my zed stays in my grans driveway just for that reason.
If the Punto owner (and that speaks for itself) is knowingly doing that to ur prized zed id confront politely but directly, check that their door has paint chipping on it photograph it and use as evidence. If they arent gonna play ball im sure we can orgainise a 350z run infront of their flat lol.
Seriously tho mate confront and get then to respray that door, why should u paint it, considering it wasnt a one off, it looks like persistant malicious behaviour,
D