Know your enemy, what sort of car is parking there?
I've compiled an easy guide for you:
Rat Car Fiat Bravo - this is a chav, a nail angled at 45 degrees under one tyre should see the scrote off
Small new Peugeot or shiny Fiesta - caution, this could be a nice young lady parking on your property as a sort of 'come on'
Berlingo - this is somebody's parent, once they've found and fixed the leak they won't bother you again
American Muscle Car - there is a male prostitute in your block, frankly they can afford to pay a monthly rent for the spot
Audi - probs a business suited hooray parking for free and catching a train - paint stripper required
T90 battle tank - the east europeans downstairs have annexed you, did you remember to vote?
Hopefully this will help you resolve the situation.