Key doe not turn off ignition

1995 Mercury Sable the key does not turn off the engine. When you turn the key to start, it engages the starter motor. But it just "spins" when you turn it back until it gets to the accessory position without any feel like it is doing anything. There are none of the usual "catches" when the key is in the run, off, or accessory position. You can remove the key, but nothing turns the car off. I had to pull a couple fuses to get it to stop. I am able to pull the ignition lock cylinder but I wonder if replacing it would do anything or if the problem is deeper so that I would not be able to replace it myself.