I was wondering, as I await the arrival of my L2i, which I plan to use with 18650's, what the best plan of action would be with a flashlight that you think is about to go nuclear.
Putting your thumb on the tailcap to try to switch it off would be suicidal, since that is a weak area and one where the pressure may be released. Besides, once a cell has gone into thermal runaway, switching off the load won't do anything - correct?
Throwing it away from you might work in an outdoors or other open area without a lot of people, but won't work in a crowded or enclosed area because of the potential for the light turning into a rocket, especially if the tailcap gives out.
If it is a flashlight with one battery tube, would the best idea be to drop it to the ground and put your foot on the center of the battery tube, to stop it from moving with minimal danger to yourself or others? Of course, if you're indoors with a carpeted floor, that's a fire risk in itself.