I agree with FurryCheeky. If your driveway and/or garage is empty then technically they are not causing an obstruction, but if your car is parked on your driveway and they are preventing your egress then that is obstruction and you should call the police. HOWEVER, their behaviour is rude, ignorant and annoying and you should do all in your power to make their life a misery. If they use the car regularly (and you don't have to go out for a day or two, let all four tyres down and then watch them spend loads of time and energy re-inflating them. Get a container of expanding foam and insert the long nozzle as far up their exhaust pipe as possible and give a good long squirt. This will prevent the car from starting and they will probably spend a great deal of money changing plugs and the coil and all sorts of other stuff before the obvious finally dawns on them. And then they'll have to fork out for a new exhaust. While they're out there trying to fix it you can go out, say you need to get off your driveway and even offer to help them push the car out of the way, all the while smirking inwardly at their predicament. If you don't fancy doing it give me your address and I'll come round and do it. Whatever, the last laugh will be yours.