Other than whats been suggested, theres not really much you can do. Well, legally anyway. You cant go clamping them as that would be illegal for sure.

A notice in the chinky is the best idea. It may take a while but would certainly get better im sure, although they'll always be some that still do it, or dont see the sign, or just couldnt give a toss.

I think it will always happen no matter what you do though, and its just something that youve gotta accept unfortunatly. Yes you can have a go at them, write on their windows etc, but its only going to be a matter of time before the shit hits the fan.