That horrid tasting polish will work on many people. But someone who is really determined to bite nails, will bite them regardless of the taste. It is a stress-releasing behavior, and the compulsion to bite is often stronger than the need to stop biting. I like the nail hardener idea, if it works, because when you are physically thwarted, it makes you aware, so an unconscious nail-biter will break the cycle.
BTW, nail biting is bad for the teeth, too. You can actually wear the dentin away.