Welcome to the forum.
First thing I'd try is disconnecting the PCV hose. Generally I'd advise against running the car with an opening near the turbo that's unfiltered, but this is only a test. Remove the hose from just in front of the turbocharger and aim it towards the ground beneath the car, make sure it's going to stay there when the engine jerks about during startup.
Now start the car, let it idle for a second or two and turn it off.
If the problem has gone away, it's a mild case of diesel run-on and it can be quite dangerous. Fitting a catch can will help, but finding out why enough vapour is making it out of your sump would be a good idea too. Overfilled? Worn piston rings? Scarred bores?
If the problem hasn't gone away, get a helper. Have them put leather gloves (gardening gloves or welding gloves will do). Loosen the positive battery cable and have your friend hold it in place. Start the car and idle it. Turn the car off. If it doesn't stop, have your friend pull the positive lead off the battery. Does it stop now?
If it DOES stop, then there's a chance your ECU is staying on and there's a problem in the ignition switch. Removing the switch and giving it a major does of electrical contact cleaner may solve the problem.
There's a couple of things to try, anyway.