Comparing this incident to an equivelent incident with cars is not the same.
If the hunt supporter tried to stop a moving aircraft, then it was an illegal interference i.e. hijack. The pilot is well within his rights rights not to stop the aircraft.
Stopping a car 'to have a word with the driver' is also against the Road Traffic Act but not considered anywhere as serious as interference of an aircraft.
Edit: I believe the gyro has a Rotax engine which stops very quickly (say a couple of engine rotations) but if the rotars were already spinning they would continue for a long time after as they are free wheeling.