I cannot get my OM617 to stop running when blocking the intake at the air filter either.
When I block the air filter intake, it just starts sucking air in through every tiny gap in the air filter lid it can find. It makes the car sound like a tea kettle, as the air being sucked in through tiny gaps makes a whistling noise.
Do what others suggested and block it off at the turbo.
Be damned careful though:
The turbo will have no problem munching through your fingers, and it'll barely slow down doing so. Using a big, sturdy piece of plywood that you can slide in front of the turbo while keeping your hands clear of it will minimize the risk of losing any extremities.
Alternatively, you can use a CO2/carbon monoxide fire extinguisher (I don't think those come cheap though...) and spray it in the intake. It'll starve the engine of air without damaging anything, but honestly: I wouldn't know where to get one. I haven't seen stores carry them, and folks say you need a 20-pound one to be successful. All I've ever found was dry-chemical ones.