Ah the old parking in the Ambulance bay trick, we get that a lot too, usually because the 200 yard walk is just too far for them. We have managed to block a few cars in before with some strategic parking, also a loud blast of the airhorn has worked a treat too. Some people just have no common sense (or the ability to read it seems). I agree with the stun gun idea too, either that or just normal guns would help in a lot of cases. In my experience they never end up complaining anyway Tom, they get home all angry and then realise that they forgot to note the vehicle number or just can't be bothered writing a letter, I am sure it will be just fine.