Hi,
I've heard about plastic nail covers that the vet can apply, with glue, to a cat's nails so that when they scratch it won't do damage. They do grow out with the nails and have to be re-applied when they do. This may be worth a try, and if it works, maybe you could learn to apply them?
This would be my first step, and then I would try behaviour modification....spray water at them when they scratch the furniture. I've also heard that tin foil paper put on the furniture is a deterrent, as they don't like to walk on it. Also blown up balloons placed in the places they scratch may help, if they pop one, it may scare them off.
Good luck!

[This message has been edited by dogncatluvr (edited October 06, 2000).]