If they are indoor cats you can keep their claws clipped short. If they go out though don't clip the back paw claws as they might need them to flee potential dangers. It's also a good idea to put some wall-mounted scratching pads around the house as they would probably rather use them than the sofa! In the US they routinely de-claw house cats but many cat owners think it's cruel and not many vets in this country are keen to do it.
Oh dear, I misread that as f***ing the sofa! Anyway, my cat seemed to prefer a sisal doormat to the furniture, which is approx �5 to buy. Also, a good slap when you catch them doing it works wonders!
Place double sided sticky tape on the affected areas cos they hate touching the sticky with their paws. You can remove this after they've transferred their attentions to a scratch post.
Also water pistols are very effective.
If the cats are only going for one spot, like the front of the arm, try putting a throw loosly over the favourite spot. Ours will only scratch where they can really get their claws in. Ours also hate the smell of orange oil. It stopped them scratching our rug.
Put a piece of tin foil on the spot while you are not there they hate the noise. There is some double sided sticky sheets commercially available that also work really well - just a bind should you want to use the furniture. Ultimately you need to train them not to scratch in the future - an empty coke can with some pennies in given a good shake whilst they attempt to scratch is a good trick - scares them to death - but works! If they are indoor only cats - trimming their claws will reduce their desire to scratch a little....