I just figured this out yesterday, so I'm glad to see that I'm not the only one who didn't know. What you have to do is bind the lightsaber to one of the buttons that you usually use for a force power (you do it the same way as you would a force power except use the weapons selection bar). Then you can press the button you chose while the saber is on to deactivate it. This works for Jedi Outcast as well. Hope that helps.