It's going to be near impossible to prevent the PF from intentionally crashing an aircraft if they wait for the proper time - such as takeoff or final approach. A quick full nose down command at low altitude simply won't provide sufficient time for useful intervention. Heck, that Atlas Air 767 pilot did a bang-up job of it during approach into Houston a couple years back, and he wasn't even trying.
Short of giving HAL complete control (at which time why even have pilots), the only way to prevent suicidal pilots crashing aircraft is to identify them before hand and stop them from getting onboard.