Have you tried providing attractive alternatives, like
http://www.catclaws.com/scratchingpads.html#4
Our cats LOVE these - they're catnip-filled, and they freak for them. And as long as we keep them fresh, they don't take a second look at our furniture. Put it right next to the furniture item they like scratching. Gradually move it away as they transfer their preference to the scratching pad.
Other options include keep-away spray, double-stick tape, and motion-sensitive alarms.
It's your cat, and I suppose a maimed cat with a home is better than a dead cat or a homeless cat, but really it would be more humane to find it a home that doesn't mind some clawing and get yourself a cat that doesn't scratch.