Poor kitty. You didn't mention it, but I assume you've ruled out allergies, whether to foods, cleaning products, pollen, or other stuff?
If the Derma-soothe that marie mentioned doesn't work, you could consider consulting a specialist. Is there a university with a veterinary school anywhere near you?
I had a compulsive overgroomer who licked all the fur off her tummy and inner thighs. She was just bald there for the last 6 or so years of her life. Is your kitty licking, or pulling out the fur, or both?