As soon as a cat realizes there is something behind that "forbidden door" there will be no stopping the urge to be there. And that door could even be the door to your closet or cellar. If a cat has been denied access in the past and even once gets past it to realize that there IS a place on the other side, their curiosity will always get the better of them. So you must choose which is more important. Allowing them outside (supervised) or allowing them to be happy inside (with less of a chance of wanting to sneak out).