What prevents a passenger from hijacking a private jet? I was reading another question on this site when I saw this answer.

As you can see in the photo I copied below, this private jet has no cockpit door. On commercial aircraft the bullet-proof cockpit door is seen as the last line of defense against would-be hijackers.

What security measures are in place to prevent unauthorized passenger access to the controls of private jets? Even though they are smaller and carry fewer people, a G650 or BBJ pointed in the wrong place can do a lot of damage.
The same thing that prevents the passengers in your car from hijacking it and using it to run people down or drive it through a mall:  It's your private car and you generally know the people that you let in to it. 

The same applies to private aircraft.