Keeping the button enabled and forcing another available category might be a good default 
(or maybe at least opening the composer without a category preselected). It’s not incredibly frequent, but we have gotten feedback about the disabled button being confusing before…
Would these moderators and admins still be able to switch to the “incorrect” category from the composer, or would it ideally be hidden there too?
Would there need to be some mechanism (a special group?) to determine who could intentionally post in the closed category?
In the event that there are no available subcategories that can be posted in, would it be better to hide the new topic button, rather than making it appear disabled?