You should never ever punish a cat in any way, they don't understand why and eventually just become nervous of you. Some people usually 'advise' yelling, squirting water, hitting, etc etc but they are wrong ! You've found out smacking her butt just makes her aggressive,she needs to scratch so you can't blame her. Scratching is natural for a cat, they need to do it for exercise to keep their leg,shoulder, and back muscles, healthy, so it's essential you have at least one good strong tall scratching post and some scratching pads around the house too.
If she goes to scratch the chair don't say a word, simply lift her gently to a scratching post, when she uses it, praise her, using her name a lot.Cats are very intelligent and she will soon want to please you.
Your cat is still very young,you only need a little bit of time and patience and your problem will be solved I'm sure.
retired vet nurse/cat rescue/35 years of well behaved cats