I had a rider that was not new to Spinning but new to my class who's form was bad and often had way too much tension. I would remind the class about proper form and safe speed during each class she was in. After a while I noticed that she was changing the way she did things and now she does not take the class she originally started in. Told me she never realized she was doing it wrong. If the person refuses to take your advice all you can do is let them ride and hope that eventually figure it out.