An airline captain doesn't have absolute authority to permit or deny anything. They are certainly empowered with various authorities, and are likely to be held to account in the exercise of those authorities. The absolute authorities are normally the statutes, orders, rules, legally mandated instructions, and directed procedures on which those authorities are based.
There may arise occasions when a captain has to vary from those mandates, however he would likely be held to account if there wasn't a very good reason for that variance. I can think of a very few situations when a variance to "authorised persons on the flight deck" might fall within such a judgement, however none of them have been alluded to on this thread.
Some countries, and their registered airlines are not subject to statutory or regulatory prohibition in this particular regard. Of those, some have operationally mandated instructions to restrict and prohibit. As to the remainder, given the current global tensions, it is unlikely they would want that information to be published.