You have a vacuum leak.  One step to diagnose is to drive around with
the doors locked and see if it'll shut off with the doors locked.  Then
drive around with them unlocked and see if it'll shut off in that
position.

It may be that some vacuum line got disconnected, too.  Poke around
inside the engine compartment to see if anything's just hanging loose.

The shutoff mechanism is down below the stop lever.  On my car, there's
a brown vacuum hose going to the valve which stops the car.  If I pull
the hose and turn the key off, it keeps running, but if I put a vacuum
pump on it and squeeze it shuts right off.  I did this diagnosis because
it takes three-four seconds to shut off when I turn the key off; I know
there's a leak somewhere but I also know that the shutoff stuff will
work better when I find that leak.  (Note that this hose should be dry
inside, if there's oil in it you've got other problems.)

Good luck,
-tom!

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