Nail biting habit

Nothing grosses me out more than people who bite their nails. My 17 year old is a nail biter and I try everything to break this gross habit. He bites his nails down so much they rarely bleed anymore. I tried the nasty Polish stuff but his issue was the taste would get in his foo d if he was eating with his hands. Like his chips or pizza. I usually slap his hand if I see him biting and his girlfriend does the same. She finds it gross too and idk how to stop it.