If the pilot flying is intent on crashing and isn't a total idiot, it's pretty close to impossible to stop it. They simply need to pick a time and place when there simply isn't enough time to intervene prior to impact. There are at least two of those times every flight...
Just a thought - what about automated flight systems that take over if a deviation from normal/safe is detected e.g. pointing the aircraft at a mountain range with seconds to spare ?