You have to buy a scratcher for him(her).

If he or she still keep doing, you have to cover the sofa with the fabric sheets or blankets, plus put the chairs on the sides. My Kelly was a great girl, only did once. Afterwards just used her scratchers. My current one is a kitten and she is stubborn. So I have to keep the door shut for my office. And covering all 4 of sofas from the top to the bottom at the living room, then 2 chairs on the sides of the long sofa. It works.