You'll either have to sand it off or use a wiring brush to remove the rust. Then you'll have to paint it or clear coat it to keep the rust from returning. Too much of a hassle if you asked me. I would just wait until the next brake job and just buy rotors that come with pre-painted hats or some type of protective coating. Same goes for the calipers...once you removed the rust, you have to coat it some how to prevent future rusting.