I dont know what can actually teach a cat not to scratch things short of hitting them, and Im not going to hit them, lol. Telling them to stop (lol like that will work anyway) and loud noises just dont work, but snapping my fingers sometimes does, because theyll walk over to me after that. They still dont learn not to scratch things though, they keep doing it. I also did mention buying scratch posts and toys for them
Also, I play with my cats, and have scars and such on my hands from where Ive been scratched/bit by them, and its really just playing gently. These have been from toms and girls, so its not whether its a boy or not.
The cats will grab your hand with their claws out, and bite you, and use their hind legs and scratch the crap out of your hand, lol. I dont know why they like doing this, but they do, and they do it just for playing with them.
Edit: For some odd reason, I have a piece of Bustaballs sig in mine... wtf
First of, you teach it slightly the same way you teach a dog things. A dog will, most of the time when you come home, go up to you, jump on you etc. They want to get close to your face, sicne that's their way of saying "Hello" and show their happiness that you're home. Simply telling it "No" in an angry voice won't make their behaviour stop. The way you handle this, is to everytime you come home, ignore him untill he stops, and when he's stopped, you say hello and welcome him. Eventualyl he'll learn that there's no use jumping up and down in excitement.
Cats however are more stuborn. They're also more self-centered and individual. Outdoor cats don't even need to be fed, they will get food and water themselves. However, you can't expect them to return home if you never feed them. So cats naturally need a slightly longer learning cycle, unless they are kittens. Kittens are easy to teach almost anything, just like how its easy to teach puppies. Children from all species are curious, and will explore new things and options. and adult stray will be extremely hard to tame, but a kitten will become tame just by having you present. Mainly because they will consider you one of their siblings, or even a parent. When they're adult. That's the largest difference between adult cats and adult dogs, as adult dogs will consider you part of the heard, while cats will consider you more like a sibling or parent.
Dogs on the other hand can be more problematic if you do things that allow them to becoem of a higher rank then you. That's things like letting it win. If it constantly jumps up in the bed until lyou say "screw it", and let it sleep in the bed, that's a victory for him. Or sayign hello to the dog and letting it rest its paws on your shoulders, which is a direct indication that you're a lower rank then them.
Cats don't have a ranking system in the same way. You teach it by being more stubborn then them. If it scratches you, don't tell it to stop. Shout "Ouch" and push your leg outwards. Don't interact with it, just ignore it untill it changes its behaviour, like mjaouing besides you, or cuddling towards your leg. You need to reward the good behaviour and ignore the bad. If she learns that scratchign your leg gets your attention the fastest, she will repeat that behaviour in order to save time. That's why you should say hi to your cat when you see her walking by, in order to show her that she have your attention, thus reducing the need to catch your attention. Just remember to say it in a childish voice, otherwise she won't know you're talking to her.
Most animals, both cats and dogs, communicate by different tones. Not actuall words. A darker, more rough voice, is used as a warning. Showing teeth is a challange and last warning for their opponent to give up and show they're lower rank then them. A normal tone is usually an indication that they want something, while a more high-pitched and gentle tone is a way of saying you accept someone and that they're doing something good. As for cats, they assume that you're not talking to them every time you don't use the childish gentle voice. Dogs on the other hand almost always assume you're talking to them, and almost always start swagging their tale soon as you open your mouth if you're facing their direction.
And yes, as Pastry mentioned, its easier to teach them while they're kittens. Luckily for me, none of our cats are bought. They're all raised by us from the time they were newly born.