From my experience, cats love to scratch on sisal rope scratching posts. Get a couple of sisal rope scratching posts, put them near your couch around the area that your cat likes to scratch. More than likely the cat will prefer the scratching post to your couch.
Get a cardboard scratching pad and train your cat with treats and praise to scratch it instead. You may also get special sticky pads for the couch at the store.
hahaha.. this is a cat were talking about!! they basicly do as they please!!. There are 2 things that can be done, yell at the cat every time you see him going to scratch the sofa. Or declaw him, however 99% of vets won't declaw a cat because of this.