Scratching is a natural behavior for cats.
This isn't exactly a revelation, since you probably have the evidence everywhere—in the tattered corners of your sofa, the shredded drapes, your frayed nerves. Though Kitty's natural propensity for scratching may not be big news, it is a fact that you'll need to take into account if you're to make any headway in winning the battle to keep her from scratching in places you consider undesirable.
You can't keep your cat from scratching.
What you can do is stop her from scratching those items you value and want to keep in their relatively pristine state.
Bear in mind Mark Twain's advice, which applies universally: Never try to teach a pig to sing; it frustrates you and annoys the pig. Translate this bit of wisdom to your dealings with cats and you'll avoid a good deal of futility and frustration.
You can't make a cat do anything she doesn't want to do. Get clear on that. And getting her to stop something she enjoys is just about as difficult. Therefore you have to think smart and re-channel her desires.
A word about punishment—Don't do it!
Cats don't understand physical punishment. In addition to it being wrong to hit your cat, punishment simply doesn't work and is likely to make your situation worse. Clever though Kitty is about many things, she won't understand that you're punishing her for scratching the couch. She will only compute that sometimes when you catch her she is treated badly. This may make her insecure and stimulate her to scratch more or develop other undesirable behavior problems.
Eventually you will break the trust and security that is the basis for your cat's relationship with you, and you will find it very difficult to catch her for any reason at all. Cats have excellent memories and hold serious grudges.
Why do cats scratch?
More to the point, why do they scratch your prized possessions? Understanding your cat's need to scratch is more than just an act of charitably on your part. It's the key to channeling Kitty's efforts to more acceptable areas.
Marking their territory
Scratching is a territorial instinct by which cats place their mark and establish their turf. Through scratching, cats mark their domains with more than just visible signs of claw marks. Cat's paws also have scent glands that leave their own special scent on their territory.
And this is why they mark the most visible portions of your house. It's Kitty's way of adding her own personal touch to your (and her) home. Her version of interior decorating.
Scratching also serves to keep your cat in shape. The act of scratching stretches and pulls and works the muscles of a cat's front quarters—a cross between a feline gym workout and Kitty Yoga.
Hey! It feels good to scratch. So give up the idea of reforming Kitty's desire to scratch. Rechannel her into scratching where you want her to. You'll both be happier.