Car problem

My cars ignition wont turn to the off position. The key can be removed and can start. The key just wont turn to the off position, it will turn to start, on, and acc, and halfway to the off position. But not there. So the battery and stereo stay on. I have to unplug the battery every time I go somewhere so it doesnt kill the battery. Ive tried shifting gears, moving the steering wheel, jiggling the key. The car is in park, it says its in park. So why wont it turn all the way off? Its an oldsmobile alero 99 v6