You could get a couple of cones to put at the corners of your car, but then some drunken fool will probably use one as a hat and put the other one on top of the car. Alternatively if you know what time he's likely to turn up just go out there and closely walk around the car and watch all the way through the manoeuver don't say anything just be there watching closely, that should be distracting and unnerving enough that the instructor will choose to take his protege elsewhere.
The official complaints process
https://www.gov.uk/complain-about-a-driving-instructor seems more for agrieved students but might be worth a try, if nothing else I'm pretty sure that back when I last had driving lessons the test centre had a dossier on the various schools and instructors and their standards.