I know an old mechanic that rebuilt engines and he kept a rag in hand whenever he worked on diesels, having been thru the factory training in Stuttgart that is what they taught then. Stuffing a rag into the air intake will kill a runaway engine if done quickly and properly. The air filter should be off as access is better with the whole filter assy removed, you will have to reach down and stuff it into the turbo inlet (best way) and that "U" shaped hose between the cleaner assy and the turbo will come off after loosening the clamp rings.
I have heard that another way to kill the engine is to cut the injector lines, you will need a big diagonal cutter or, better yet, a tool made for cutting wire rope (that is what I would use, fortunately I have never had a runaway after changing the shutoff actuator). Either way, the action would have to be quick!
If the engine takes off and revs towards infinity it will self destruct before about 10 or 15 seconds.
DDH

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'95 E320 Wagon my favorite road car. '99 E300D wolf in sheeps body, '87 300D Sportline suspension, '79 300TD w/ 617.952 engine at 367,750 and counting!