In my aerobics classes, I'm often uncomfortable singling someone out. I'll look straight at them, give a general correction to the class a couple of times. If they're not listening, well... maybe I should be more direct. The higher the chance of injury, the more direct I usually am. When it's just a matter of getting the benefit of the class, I can't spend all my time focusing on one person who doesn't want to pay attention.

It depends on the student, too. If it's someone I've already corrected a million times and seen other instructors correct, and they still ignore the instructions, I'll just give a pro forma instruction so the rest of the class doesn't imitate them.