Nail polish
you will get an awfull taste when you do it and stop automaticly
Depending on the nail polish this can be poisonous. Don't do it. Nail Polish is not safe to consume.
You can get "Nail Biting Solution" instead, which is a safe polish intended for consumption, but tastes horrible.
Check your local pharmacy in their nail care range.
This might not be of too much help if you don't put your nails into your mouth, but if you are then it should disuade you from doing it if it's an involuntary action.