Cats don't like citrus, if you add the juice of half a lemon (or other citrus fruit if you prefer the smell) to a litre of water then spray it on the surfaces you want to protect it will largely keep the cats away. You'll need to reapply fairly regularly.

You can also put strips of double-sided tape on cardboard sheets and leave them on seats etc. while you're out. They don't like the texture of the tape or it grabbing on their fur so will avoid those surfaces.

If you see the cat doing something naughty, giving it a quick spritz of water spray will really annoy it, but it will quickly associate bad behaviour with a damp coat and eventually stop.

To get the message across, you need to be consistent, and always punish/discourage for the same behaviour. If you sometimes let them on the sofa and other times don't you'll just end up with a confused cat.