yes a lot of kids have been pinching these to put on their pushbikes... the only safe way is to buy the metal valves then you can safely nip the hexagonal cap with a spanner.. but again grease the thread first to eleviate ceasing on.
so suprising their initial attitude to say they are a nissan dealership. would have expected that attitude off an independant or even back street trader. best of luck with the repairs bud.