It isn't illegal to park your vehicle on someone else's driveway. But it is frustrating and can cause friction between neighbours.

In the first instance, I would suggest speaking to the owner of the vehicle and asking them (politely) to park somewhere els. If that fails to solve the problem, you may want to consider fitting a lockable gate or bollard (which you can unlock to access the driveway when you park your car).

In a worst-case scenario, you could take legal action against the driver and pursue a civil case of trespassing in order to have the vehicle removed by a court.