Screening for Malicious Intent
Armoured doors, door codes, cameras, video surveillance, live telemetry, 2 persons in the flight deck, 200ml of yoghurt, toothpaste, soup etc etc.
I believe all are missing the point. If one of the people at the controls of a conventionally controlled aircraft (e.g. B737, B757 etc ) has malicious intent, he/she could have the wings off before the other pilot could do anything to stop it. I don't know whether the flight envelope protection on an Airbus would prevent the above.
So, truly the only way to prevent a recurrence is more rigorous psychiatric screening of flight deck personnel.
Brace yourselves folks!