Parking over neighbours drive way... Again, again.

couple park their cars across old man's driveway while they go to the beach. old man likes to use this space to park his van, so he parks over my driveway (blue car, I said it's fine) to block them in and confront them about it when they get back. he doesn't use his driveway to park (must of the time) but he uses the dropped kerb space to park, like a private parking spot. which the couple have parked on. and yes there is a dropped kerb.