"A cat cannot be trained by a human and will never respond to discipline of any sort, leashes......etc"
Sorry, but you are very, very wrong. I have walked cats on leashes for more than 10 years. If they go outside, they are on a leash. All my cats have been trained where they can/cannot sit, jump, scratch, etc. It just takes a little investment of time upfront, and then you're done. That word NO and the water sprayers mentioned earlier are very important!
I've never had a cat that's not trained. I wouldn't want one that wasn't. To keep your cat from scratching where he is not supposed to, you will have to get a scratching post or posts or objects that are for the cat alone to mark his territory. That's why they go back to the same spots, to mark territory. Some cats will naturally adopt the scratching post, but others will not. I think sometimes people expect the cat to realize that hey- I bought you a scratching post - don't you see it? Just like everything else, you have to teach them that the object is theirs to scratch at will. How else will a cat know that he is allowed to scratch on some things and not others?
Get your scratching post. In my experience, those covered with sisal rope work tons better than the ones covered with carpet. And not the ones that are 'straight up' in design, get them on an incline or flat so the cat can stretch while scratching. Also the carpet ones are kinda like your carpet, your lazy boy, your couch, etc. Talk about confusion! At pet stores, they also have these ramps or mats made out of corrugated (sp?) cardboard. These work great and are cheap. I've never had a cat that doesn't like to scratch on those. They usually come with catnip - sprinkle some on to attract the cat to it. (some cats are not responsive to catnip). I once had a 4x4 piece of plain old wood that my two cats used for 20 years. When they died, I got rid of that eyesore and moved to the sisal rope/cardboard ramp combo.
After you get your post, you'll have to be diligent. When you see the cat scratching where he isn't supposed to, pick up the cat, take him to the post, and move his front feet against the post. Some words of praise here don't hurt. (ha) It usually doesn't take very long to establish this pattern. If you have something that the cat is determined to scratch, take some double sided tape and cover the spot with that if you can, or a plastic tarp if needed for a while on the couch arm, etc. Cats hate sticky on their paws. I had to plaster my kitchen sink one time with double-sided tape! (I hate cats on tables, countertops, etc.)
I only have indoor cats now. I used to let mine go in/out as they wanted. Several years ago, I brought indoors 2 cats that were feral. One had lived outdoors for 14 years, the other for 9 years. They were wild. (and incredibly easy to train, I might add. If I didn't sit on it, they didn't jump on it.) After having them indoors for a few months, taming them, training them, etc., one day I had the door propped open because I was moving some stuff outside. They both rounded the corner, looked outside and took off for the back of the house like little bullets. "I'm not going back out there" was the message I received from that! Also, we have coyotes everywhere now, and for me, it's easier to keep them healthy and I don't have the added worries. I have two shelter cats now that were adopted as kittens. They have never been outside and have no desire to go out. My other cat was rescued from a dumpster, so we do the leash in the summer months on special occasions. So I'm lucky in that respect.
Oh - wanted to mention that after working with animal rescue groups for many years, a lot of cat rescues are declawed cats. Some cats are okay after declawing, but some also develop behavioural problems afterwards due to the pain or discomfort they are in. So, the owners give them up.
Teach your cat - it's really not difficult, just very repetitive. Animal experts say that cats are harder to train than dogs, because they are smarter. I don't know about that, mileage varies, but it just takes time like other things that are worthwhile. It will pay off in the long run!
Sorry for the small novel and Good Luck to you and your kitty!