KenK,
Put your Photon Freedom into "signal" mode by pressing it rapidly four or four times until the light only comes on for as long as the switch is pressed. If something turns it on in this mode it will only stay on for as long as there is pressure on the button. When you want to change out of signal mode, just hold the button down for a few seconds until the light goes off and you are back in normal mode.

It won't prevent the light coming on for short periods but it should stop the batteries from draining.