Personally it's better than declawing; if you like cats, don't declaw them EVER if they go outside. If it is a house only pet then you might find it acceptable, but still...
First of all your cat knows it's not allowed, it simply doesn't care. Our cat looks shifty when it's done something bad, will stop and lurk at the entrance to our bedroom if the door is open (she's not normally allowed in), even if she's unaware she's being watched, building up her courage. So what we have here is a failure to communicate.
However, I don't know if you've tried cat.
Cat is a language. Although cats can develop a largely intonation-based volcabulary and learn to respond to certain words (our cat knows 'down', 'out' and 'cat' and will respond even when not looking our direction), as we are the smart ones, we should learn to communicate with them.
I would basically wail into that cat with a squirty bottle of water at first. When it scratches, chase the little shit and yell and spray. Escalate if required to an aerosol can of something benign enough to spray at the cat, preferably one that makes a loud noise. Believe me, cats associate unpleasent sounds and results with actions, and will change their actions.
Our cat is a terror - it attacks people, literally. It only attacked me once. In such a situation I do not think poor widdle puddy tat. I 'kicked' her across the room (as in hooked my foot under her tummy, lifted her off the ground, and without actually hurting her, lofted her with my foot with sufficient velocity for her to land on a couch on the opposite end of the room). This REALLY impressed her, but THEN chasing her down, grabbing her scruff in me teeth, and holding her down on the floor for five minutes whilst growling (although not huting her in anyway) made it clear I was car not to be pissed around with.
The cat thereafter decided I was boss, much to the chagrin of my fiance.
So, now you have a sort-of-solution in place, just go ape-shit when she claws where she's not to. Don't hurt her, but don't be afraid to be terrorise the little shit. She'll respect you for it.