One way to stop cats from scratching furniture is to make the surface of the furniture unappealing to them while also providing them with plenty of fun things to scratch instead. Sticky tape, aluminum foil or plastic wrap can deter cats.
Scratching is a natural and healthy behavior for cats, so they need to have appropriate outlets for it. Individual cats often have different preferences, so you may want to experiment with different types of scratching posts. Rubbing the surface of a scratching post with catnip may also help attract the cat, as can placing the post near the furniture the cat likes to scratch. Once it gets used to scratching on the post instead of the furniture, you can remove the sticky pads or other deterrents.
If that does not work, keeping your cat's nails trimmed so that they are dull may help reduce the amount of damage. You can also try applying nail caps, such as Soft Claws. These strategies do not stop the cat from scratching furniture, but they may ease the problem by preventing your cat from causing any damage.
Declawing cats is also possible, although it is illegal in certain areas. This surgery involves removing part of the bone of the cat's foot, so it should be treated as an absolute last resort if it is considered at all. It is also not suitable for outdoor cats, because they need their claws for protection.