fiv4fghtn: The recommended ways are to let it run out of fuel, pinch the fuel line (as near to the carb as you can) or cover the exhaust outlet.

Basically, the safest way is to let it run out of fuel, but this isn't always appropriate. Pinching the line will make your motor run lean for a few seconds, and you'll most likely have to give it a quick prime before you start it again. Covering the exhaust can flood the engine but is the quickest way to stop it (save for the flywheel method).

Personally, I stick to pinching the fuel line. A few lean seconds isn't gonna cause the engine any harm and I'd rather have to re-prime than undo a glowplug to sort out a flooded block, especially with these rotostarts ripping the one-ways to pieces!