Not saying it would work for everyone, but I quit by learning to play the guitar.

For the purposes of strumming I needed longer nails on my right hand and shorter nails on the left. So I stopped myself biting on the right and allowed myself the left for a while.

Once I had got the right under control I started aggressively trimming the left so there was nothing to bite.

In the scale of things though, nail biting is a relatively easy habit to break. I doubt this technique, or a variant of it, would have much of an effect where crack addiction is concerned.