If you allow the user to see the Navigation pane and the Ribbon, the answer is "you cannot do this."
If you have a system in which you have a switchboard form that is ALWAYS there in the background, and if every action taken on a form resets some internal timestamp visible to the switchboard, then you could have an OnTimer event in the switchboard that checks every minute for how long it has been since the user last did anything. You could make your decisions from that information, and the switchboard form could do an Application.Quit for you if that was what you wanted to have happen.