What is happening is that the throttle is remaining partially open when the engine is shut down, which gives the hot engine sufficient fuel to run without a spark from the spark plugs. Most hot engines have sufficient carbon build-up that remains glowing red hot and acts as an igniter for the fuel. The solution is to make sure that the throttle closes completely when you turn off the ignition switch. Check the throttle stop and make sure that the fast idle on the choke or the "bottom stop" isn't what is stopping the throttle from closing.