I believe they call that effect "Dieseling" when an engine keeps on trying to run when you turn the key OFF. The culprit behind it is usually carbon deposits on top of the piston. While running they glow red hot and even when the spark plugs are OFF, the hot, glowing carbon on top acts like a diesel engine in a way and fires the fuel. You might could get some factory approved solution to run through it and dislodge some of that carbon. Take out a spark plug and see if you can see a heavy carbon deposit on top of the piston. Other than the carbon-cleaning chemicals, the only other option is to pull the heads, which I wouldn't do. When I have an engine diesel on me, I just throw FULL CHOKE on it with the blades going and let it die. Hope this helps some.
