The onstar box is under the driver seat. It has two antena cables (wi-fi & onstar). They have safety plugs so use a small screwdriver to pry them off. (Easy to figure out by looking at them) That will disable comms but the unit will still be doing its thing. Unplug the other two "power" plugs and it will just be a brick under your seat. There will be no engine lights, no codes and nobody will be able to track you or hack your spark.