we eventually stopped the engine using a pillow stuffed into the intake.

He had this same problem later on, on a long cruise, and again used the same stuffing solution.

If there was any sort of manual override, it sure as heck wasn't obvious.

I followed the solenoid connection later and trouble shot the problem (it was a solenoid failure). I saw no other way to cut off fuel. (well, there was another way... turn off the electric fuel pumps... )