When I was teaching aerobics, I had a woman who would come to my class, wear a Walkman (this was the '80's) and then proceed to sing loudly to the music that she was listening to. She also dressed kind of provocatively. None of the other instructors would say something, so, finally, after asking her to stop, which she didn't, I told my boss. She stopped. I've seen all kinds of rude.
Some of the gyms around here take the older models of the spin bikes, or even just 2-3 and put them in the cardio/workout area, so people can use them when there is not a class. Personally, I just stay home and do the trainer if that's what I want; I only go to spin if I feel like being in a group.