First off, there's no reason to ban a half-inch beard for security purposes. That's not thick/long enough to hide a weapon in. So if there are legit security concerns, it should limit length, not ban them outright.
If there was a religious requirement for a long, thick beard, well I don't know of any religion that requires you to meet a specific length for facial hair, so I'd say determine a reasonable length as the cutoff for safety purposes and don't allow anyone to grow it out further.