Scratching objects to mark them is a normal cat behavior. This behavior appears to help in claw sharpening and to provide some stretching exercise for the cat. Outside cats tend to scratch wood objects. Some cats prefer a vertical surface and some a horizontal surface. Noticing which your cat prefers can help in dealing with problem scratching. Cats are also picky about the texture of the objects they scratch. Some become very fixated with one particular type of fabric and will not scratch any other surface texture. This can also be important in retraining them. Cats like to pick a prominent spot for their scratching, since they are partially doing this to announce their presence to the world. Putting their scratching post away in the corner of a seldom used room may not fulfill that need. It does no good to try to keep a cat from scratching furniture if an acceptable substitute is not provided. Most cats prefer a longitudinal texture (like the stands on a sisal scratching post which run in one direction, mostly). The scratching post should be at least 1 foot tall ,or long, depending on its orientation. Many cats like a bare, soft wood as a scratching post if fabric ones are not attractive to them. The sturdier the construction, the more likely it is that the cat will like to use the scratching post. If it can be placed in a prominent site that helps a lot, too. If the middle of the living room is not acceptable, putting the scratching post in a prominent site in another room might work ---- but away in the basement probably won't. Paying attention to providing an adequate site for your cat to scratch helps a great deal. When they still won't cooperate and leave your favorite furniture alone, it is necessary to discourage the behavior.
If possible, it is best to remove the object the cat likes to scratch while attempting to change the behavior. If removing the object is not possible, it can be helpful to cover it with plastic, which most cats avoid. Consistently punishing the cat whenever it is seen scratching can be helpful. Spray from a squirt gun is a good deterrent for most cats. Then take the cat to the scratching post and even run its feet up and down the post in a scratching motion (gently!). When you leave the house, make sure the cat is not allowed access to the room with an object it likes to scratch, if possible. Most cats will learn to use a scratching post if you are persistent in attempts to alter the behavior.
If your cat will not stop destructive scratching behavior, declawing is an option that may provide some peace in your household. In studies of this behavior, worries over behavioral changes or long term complications have been unsubstantiated. Most owners feel that no behavioral change occurs after declawing. While this should not be the first choice, it is an option. Declawing can be the difference between being able to have a good relationship with your cat or deciding to consider much less acceptable alternatives such as euthanasia or abandoning your pet. When that level of frustration is reached everyone is better off with the decision to declaw. Alternatives are clipping the claws regularly so they are too dull to cause much damage and using Soft-Paws or beads glued to the claws to prevent damage from them.