Disabling Onstar is not a task that is as simple as pushing a button. Onstar is a device that permits the vehicle to be tracked and, if something happens to the vehicle, allows an operator to conact the vehicle. For some people, there is discomfort with Onstar having the ability to track the vehicle. To disable Onstar, it requires the device to be completely disconnected from the vehicle.
Open the trunk and lift the liner that is over the spare tire. Place this to the side and remove the tire. If there is a nut that needs to be removed to get the tire out, turn it counter-clockwise. Remove the tire and set it aside. The frustrating thing about the Onstar is that they placed it in such an obscure place that requires a considerable amount of moving of things to access it.
Now that the tire is out of the way, remove the trunk still plate which is where the tire rests on. There should be four screws near the four corners of the trunk. If there is a cargo net in the trunk, unscrewing the screws that hold the net in place is all that is needed to unscrew the tire plate. They are the same screws. If there isn't a cargo net, try and find the screws in the corner. Set the plate aside and then pull the top right side of the trunk shell back. The Onstar device should be visible there. It'll be a small metal box that has the Onstar logo on it.
Unplug the three different wires that are connected to the box. These provide power and communication for the Onstar. Unplugging them will disconnect all power from the device and therefore, it won't be able to transmit information.
Now that it is unplugged, put the shell back in place and then put the plate in the car. Screw the screws in, not forgetting the cargo net if there was one in the car. Put the tire back onto the plate, tighten the nut clockwise if it needs to be and then put the liner over the tire. Make sure it is secure in place and then close the trunk. If all wires were unplugged, the Onstar will be successfully turned off.