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 behaviour 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.
Fortunately, there is a simple alternative available for you and your cat. It consists of a good, sturdy scratching post covered with strong material and lined with catnip. You can make one yourself or it can be purchased. With close attention and a lot of encouragement, your cat can be perfectly happy scratching on his own furniture rather than yours. The following is a list of countries in which declawing cats is either illegal or considered extremely inhumane.