Just replace. I know it’s not what you want to hear. If it’s superficial you can get the surface rust off usually just with novus 2 and a rag and wax them and put ‘em back in. As has been mentioned once you get rust it will come back over time most likely. Expect to change the balls once a year or every 6 months if you play ALOT. It’s not just the rust. Balls get nicks/scratches that will cause wear. You can polish but never remove those nicks/scratches if they are deep. Not worth the time/effort. Simply replace as needed.