You can use a hemostat to reach in to the fuel line to pinch it closed. Having that thing locked on your line when you choose to start it is no problem. It'll be hanging out the front of the cowl and the prop can't get past the sucker.

I had two models that siphoned, one inverted, one at 9 o'clock. Finally got fed up one rainy day and lowered the tank in one and got rid of the uniflow routing in the other.