I've heard that cats don't like citrus smells. Maybe you could spray something like that on there?

I was at PetCo today and saw this scratcher thing that you slide next to the couch (there's a metal "foot" you put under the couch to keep it stable) and it deters cats from the couch and entises them towards it instead. I didn't see it on the website though.

Does spraying them with water help at all?