There is a spray you can get called 'Get Off!' spray which is citrus scented and designed to be sprayed on carpets, upholstery, walls etc... I agree that the best response is a firm loud 'No' command coupled with loud hand claps. You can also get rattan mats that are designed to be hung on walls - you could maybe put those up over the wallpaper? It is a hard problem with cats because it is such a natural instinct for them to scratch. You could also try providing more tempting surfaces for the cats to scratch - normal scratching posts are made of carpet. But cats love to click their claws on wicker - try leaving out a wicker basket for them. Also they like to scratch wood. You could try to get a big piece of driftwood for them. If they like the alternative enough, they might leave your walls alone! Good luck.