1990 Buick Regal how do I shut it off?

The gear shift needs to be in the park position to be able to turn the key to the lock poition. Engine would turn off at the ACC position.

If the gear shift position is correct, the interlock system has a fault and it could be the key in adjustments are faulty.

If engine is not shutting off and you can turn to the ACc position, you have an ignition switch that is stuck in the OB pisition. It could be a faulty ignition switch or a worn key inside the key cylinder that is not turning the switch back to where it is.

Remove the steering column cover and unplug the ignition switch wire harness.

Did the engine stop? It is not a good idea to disconnect the battery and alternator while engine is running. You could cause a lot of damage is something goes wrong.

Pulling out the fuel pump relay until engine stops and then disconnecting the battery would be safer.