We had the same problem, abeit it was a fabric sofa, and bought them a cardboard scratching board (just lies down on the ground, but it has catnip in it, and they were IMMEDIATELY drawn to it). It's official name is Smarty Cat Super Scratcher. It's one of those that isn't too expensive...look here http://www.cat-toys-superstore.com/en-us/dept_20.html
. I have to say, too, I've had tons of different scratchers, and NEVER had such wonderful success as I have with this one.
GOOD TIP: For the first couple weeks after purchasing a scratcher, when you see your kitty scratching something undesirable, pick them up, put their claws out, and have them "scratch" on the proper place...the scratching post. They'll get the idea!! :o)