I can't think of any offence you could be committing.
Wilful / unnecessary obstruction is a possibility - but usually applies to obstruction of the highway, not a private parking space. If you were parked on the highway to block this vehicle in, then you could be liable for a charge of obstruction, but that doesn't seem to be the case.
Another possible is false imprisonment - but this would only apply if you stopped a person moving freely, not his or her vehicle.
I would advise you to speak to the driver in the first instance and ask him not to park in your space again . If this is not possible, leave a note on his windscreen.
Put a sign in your space - private parking, any vehicles parked in this space may find themselves blocked in. Or something like.
But if if has been going on for sometime and you feel you need to do something short and sharp - block them in.