I had the privilege of serving as a course marshal last August at the Classic TT. On the first practice day they were short of marshals and could not start practice without a full staff, so I responded to their appeal for volunteers. Of course, I ended up on the mountain (Mountain Box specifically) where the temps were low and the wind was gusting 40 mph or so.
They do indeed take the rules seriously. After all, lives are literally at risk. When the roads close the status of the course changes from a public road to an official race course. Marshals have the power of arrest for anyone refusing to comply with the laws or the direction of any Course Marshal. They did advise us not to attempt an arrest unless we absolutely had to, recommending that we call police instead. From the pictures I saw it appeared that the man was actually restrained by the Marshals until the police arrived to arrest him.
I distincly remember the gravity of the feeling when the lead marshal turned to us and said "the roads are now closed and are now an official race course."
Me on the left with Cindy and Alex McLean:
With Lorraine Crussell and "Ad" preparing to serve as refuelling crew for Dave Crussell: