How to handle motorists' dangerous behaviour with an impassable group?
Don't create the situation in the first place. Every large event I've ever been on would either have the roads closed down, or send the riders out in waves so as not to create a situation where a car would have to pass so many cyclists at once.

Obviously the guy in the car was in the wrong, but there are still things that can be done to prevent the situation in the first place. Forming a defacto blockade on the road isn't really in anybody's best interest.