The problem you have really mate is that there will be a streamof different people doing it.

You tell one, and another is right behind to do it. For this reason, I would go with a nice BIG sign saying "YOU WILL BE INSTANTLY CLAMED IF YOU PARK HERE"

This will be enough to deter all but the biggest c*ck heads who will park there no matter what.

Dojj, tell them what you did to the school run mom - classic!!!!