Many engine manuals reccomend pinching the fuel line with a pair of pliars, but i find that the easiest (and quickest) way to kill an engine is to plug the exhaust. Just put your finger over the stinger, and in under a second the engine will die. As the finger is only in contact with the hot surface for a split second, it is unlikely to burn.

Just to reduce the chance you can lick the finger you use, or use the heel of a shoe.