Diesels are stopped by cutting off the fuel. Gas engines are stopped by cutting off the spark. On my old Universal, the engine was stopped by moving the throttle past idle to cut off the fuel. On my current Yanmar there is a separate fuel stop handle to cut off the fuel. Stopping a running engine by activating the decompression release as sawingknots suggests is a bad idea. Below is from the Universal manual.
"DO NOT USE THE DECOMPRESSION LEVER TO STOP THE ENGINE!
This practice could seriously damage the exhaust valves."