The problem is the ignition lock cylinder has come apart inside and this is why it can not be turned back to the off position. You can pull the POWERTRAIN RELAY NUMBER 29 in the under hood fuse box. Remove this relay and the engine will stall. Then you will need to remove one of the battery cables to turn off all the electrical power so the battery does not drain. You can restart the car by installing the battery cable and relay, then turn the key to start.
You will have to have the ignition lock cylinder replaced to repair this problem.