You want to make the couch as uninviting as possible until the cat loses interest in it.

In my experience, most cats really hate the smell of oranges. Perhaps using a strong orange-scented spray on the couch might help. (Note: Some cats don't mind the smell.)

You can spray the cat with a squirt bottle whenever it comes near your couch, though this will only work when you're nearby. (But, again, some cats actually like to have water sprayed on them.)

I've also heard of people using plastic chair mats where they don't want their pets to go. If you place it upside down, the sharp little pegs (intended to hold the mat in place when used on carpet) should stick up and irritate your cat's feet.

You may have to experiment a bit to see what works best for you. You will probably want to try a combination of things, just to reinforce the lesson.

My cats used to enjoy scratching the couch because we were always on it and they like being around people. So we got a cat pod (for free, my old roommate works with the guy who makes them in Dallas) and put it, along with their other scratching post, right next to the sofa. That gave them an alternative that was close to where we sit. Cats really love attention, so scolding is still attention in their world. Putting the scratchers close to the couch means we can redirect them to what we want them to scratch by scratching it ourselves (they love that), thereby giving them plenty of attention and play time when they do the right thing. Our cats love all the things they're supposed to hate -- spray bottles, citrus scents -- so the only thing that works is positive reinforcement and regular nail trimming.

Cat Scratch Guard.com

I had cat scratching the couch problems for years. Nothing worked so I came up with my own idea. Nearly invisible, easy, cheap and I'm done.

http://catscratchguard.com/