Firstly you could open it up using the tool supplied to change the propellers and then disconnect the battery connector.
You then need to find the fault which could be the switch. If you have a multimeter check the operation if the microswitch which operates on the arm. It should be open circuit when the arm is open and closed circuit when it is shut. I know that it sounds backwards but that is the way it is.
I opened the controller and tried reconnecting the battery, that is as far as I can go. I do not have a multimeter. The controller still won't shut off when I reconnected the battery so I just left it disconnected.