Danny’s advice about not using your hand should be heeded, guess how I know!  I had to smother a diesel once that broke all of it’s mounts plus the coupling so was laying on it’s side still running.  It was laying on the controls and the injection pump so the controls were jammed and there was no way to reach the shutdown lever.  This was a delivery nightmare situation and I don’t think the boat even had a fuel shutoff or at least not one that I could find.  What I learned from trying to strangle a diesel is that it is not easy.  I put a towel  folded multiple times over the intake and the engine did not even seem to change rpm  (it was at idle thankfully).  What worked was to wrap the towel as tightly as possible, shove it as far into the intake as I could, then twist the towel... and then add the now bruised hand over the top  to finish it off.   As has been mentioned, you sure would not want any part of the towel to get into a turbo or maybe under an intake valve so using a towel or rag might be a bad idea.  In retrospect a firm pc. of rubber sheet or something that could be placed over the intake would probably be a better idea.  

Best,

James

SV Sueno

Maramu #220