You might also think about trying Feliway spray. We purchased some at Christmas time to spray in our car (since our cat came on vacation with us). One of the suggested uses is to help deter scratching - you just spray the spot once a day (they suggest for at least a month).
We've been having occasional trouble with our cat scratching the sides and back of our couches - luckily, the fabric hasn't allowed her to hook her nails into it (yet). It's hard to say whether or not using the feliway is actually working - we only use it when we see her scratch - maybe once or twice a week. Once we spray, she doesn't usually go back to scratching for a few days but maybe removing her from the spot to apply the spray is what is actually detering her?
I'd love to try a once a day routine to see if it would stop the scratching all together, but the spray is really expensive to purchase here in town ($65 at the one store that actually carries it!). We purchased online for about $20, but we've been trying not to overuse it for scratching since we purchased it for another use (ie, car rides). It's funny though, when she goes near the spot soon after I spray it, she gets a bit of a confused look on her face then usually just walks away. It must be doing something - at least she recognizes that it's there. If you can get it for a good price, you could at least try that?