Think its time for some of those "you cant park" spotted cards metioned on here before. Anyone does it to you, wham, slap em with it. We should make it out of the same tickets they used at my Uni, which are impossible to remove without making a mess, super strong glue and really thin paper
Like this....
http://media.ebaumsworld.com/picture/an ... arking.jpg
Click to view (warning it has bad language!)