I might get a nip from one of my cats when they want my attention or have had ENOUGH attention. They nip to play, and my late cat Lucy would nip my head if I was getting too rowdy, like I was one of her kittens!
That said, my friend has a cat that bites and scratches for more sinister reasons, and I've never figured out how to stop her. My friend doesn't care, and I just ignore her when I go there. She stalks me, but doesn't bother me if I ignore her or hiss when she gets too close. If she were my cat I'd certainly be looking for a better solution, too.
Have you spoken with your vet?