What I see as a deterrent is this:

Joe high school(could be anybody, but my experience is the younger they
are the closer they are) in his Camaro pull up to your bumper at 60 MPH.
An electric eye set to a certain distance is tripped and your license
plate flops down, and out comes this retractable hand/claw type thing.
It snakes its way into the engine compartment of the offending
vehicle(fuzzy logic would need to be used here for the arm to find its
way around the engine. When it reaches the engine the arm proceeds to
remove the spark plugs rapidly, and then retracts itself and the license
plate closes, with the arm lying in wait for the next unsuspecting
victim to approach your bumper.

Lying in wait,

David
74 Thing