If you want some help, your going to have to give us a little more info. The techs on this site are good, but do not have an ESP. Remember you are standing next to the unit and we are thousands of miles away. Paint us a picture of what you see and are doing now to shut it down. Including model and serial numbers of the unit and engine. Quick guess would be a bad switch or ground.
Could be a lot of things. Bad key switch, bad time delay module, corroded connection, wrong heat range spark plug, carbon hotspotting.
When you turn the key off what exactly happens? Like Rivets said. The more we know the more we can help.