1991 Cadillac Seville

There is no turning it off. More than likely, the ignition lock cylinder has failed. There is small wires that lead up the column to the cylinder. Over time, through rotation, and use, the wires eventually break at the cylinder. The preferred method of repair, is replacement of the cylinder. With a little shadetree knowledge, you could install resistors in the same value as the key pellet, at the connection at the base of the column, bypassing the key sensing feature.