No idea how long ago this was posted: my action would be to call the local news agency and local TV stations, explain the situation and see if you can set up a news-mediated confrontation with the offender when he/she returns to their car.

Also call the police - supine and useless as they tend to be in matters of this nature, the prospect of Press/TV action might get them off their butts to oversee matters and prevent things getting out of hand.

And then: name and shame! Make up posters, put them in shops local to the offender's home. Take photos, write to the local press, all that.

Or - pay a removal company to have the car moved, then immobilise it until the offender has reimbursed you for the cost of doing this. Sue them if necessary, or get an injunction preventing further trespass.

But under no circumstances take any action that's going to put you in the wrong, don't damage the vehicle or assault the driver.