Or just rip the airfilter snorkle off and stick your hand over the airfilter inlet stopping air getting into the engine and therefore stopping the engine from running.
Gonna need pretty big hands/palms imo.
Well in theory you could stop it, if you cut off the oxygen supply (which is needed for the fuel to burn) as devonmikeyboy suggested, by pulling off pipes and blocking the air inlet.So you cant stall it, as there is another fuel source.
The real question is, does it run out of oil or blow up first? lol. I dont want to find out!
No, afraid not, the valve is vacuum actuated via a little vacuum switch, and only closes for a few seconds at engine switch-off. Default position is open. Has to be, really, or you'd never get the engine started.