TWO PEOPLE ON THE FLIGHT DECK AT ALL TIMES DURING FLIGHT. Facilitated by a member of cabin crew remaining momentarily on the flight deck (as they would during single pilots ops in the event of a pilot incapacitation) during toilet breaks etc. It's not new. It's already applied. It should be mandatory. Nuff said!
Whats to stop the remianing fligth crew member using the crash axe against the FA... or in the USA some of the flight crew carry guns so they would just shoot them. If they have intent they will find a way.