Three prong approach:

1. Use a spray bottle. Use half and half, water and WHITE vinegar. Spray the furniture where they scratch. They can smell this, you can't. It deters them.

2. Get some nifty tall cat trees / climbers / perches. Rub lovely catnip on them. It attracts the cats.

Be sure to apply the spray and the nip at the same times, several times a week at the beginning.

3. Double sided sticky tape. Unsightly but heck, easy to rip off when company comes.

And yup, as Donna / Moosmom said, cats don't know this furniture yes, that furniture no. Keep them from scratching ALL furniture and restrict scratching to THEIR pieces.

Good Luck; sounds like you have a bad habit to stop, rather than no habit and new behavior to instill.