Was the wardrobe door still functional in "Prince Caspian"?
Well, don't forget that these gateways operated at Aslan's command. It's very likely that Aslan "closed" the wardrobe after it had served His purpose - that of bringing the Pevensies into Narnia. I believe it's in Prince Caspian, when He's explaining the history of the Telmarines, that He alludes to how the chinks and chasms between the worlds can cease to operate.
So why Aslan didn`t turn the rings off?
He could have - but then He may have just left them as they were, and had His agents handle them properly. Remember how He explains to Digory that "Things always work according to their nature." Jadis took the apple that she shouldn't have, and reaped the benefits (immortality) but not the fruits. Likewise, Uncle Andrew "messed about" with the Atlantean dust to create the Rings, and according to its nature, they worked.
But the proper nature of a wardrobe is not to act as a gateway to other worlds. That is, if you will, an inadvertent feature, something that is in addition to the normal behaviour. Remember, the wardrobe was made of wood from the tree that grew from the core of the fruit which Digory brought back - hence its roots really were in another world. It was something that stood on the threshold, and thus would not be expected to behave as other things. Remember that nobody could use it to get to Narnia if they were intending to use it to get to Narnia.
Aslan could open doors when and how He would. Recall how He opened the door in the wall at Experiment House when He was summoning Jill and Eustace in Silver Chair. Things that belonged to worlds behaved according to their nature in those worlds (e.g. the fruit, or the dust from the Wood). Things that stood on the threshold offered Aslan more discretion.
But the proper nature of a wardrobe is not to act as a gateway to other worlds. That is, if you will, an inadvertent feature, something that is in addition to the normal behaviour. Remember, the wardrobe was made of wood from the tree that grew from the core of the fruit which Digory brought back - hence its roots really were in another world. It was something that stood on the threshold, and thus would not be expected to behave as other things. Remember that nobody could use it to get to Narnia if they were intending to use it to get to Narnia.
Aslan could open doors when and how He would. Recall how He opened the door in the wall at Experiment House when He was summoning Jill and Eustace in Silver Chair. Things that belonged to worlds behaved according to their nature in those worlds (e.g. the fruit, or the dust from the Wood). Things that stood on the threshold offered Aslan more discretion.
I wonder why? As I recall, they were considering to use them in the LB...