Why don't you combine the two items, by training him to use a different method of asking to go out, like the old "door bells" idea?
Right now he knows that if he scratches and claws at the door, he'll eventually be let out; you've essentially trained him to know that scratching is the way to success... so instead of training him NOT to scratch, teach him that SOMETHING ELSE will work, and if he rings the bells, someone will come and let him out.
Of course, you'll have to stay right on top of it at first, making sure to let him out as quickly as possible when he rings the bells (once you get him started using them, that is). The bonus is, learning something new will also help stimulate and occupy his mind.