Rat 'em out; 'hang 'em high'. There's a safety aspect for both bicycles and vehicles using the road. You don't mention where this was, so you're going to have to find out who has jurisdiction - a city, the county, the state, or the feds (SMM Natl Recreation area), and report it to them. And did it look like they might be doing a professional filming gig? That usually requries permits, which cost $$$$, and knowing how tight govt agencies are for money, the local jurisdiction will probably go after them for any fees they were supposed to pay.