On-Star can be defeated ,, simply disconnect the antenna or fuse. (the FBI can listen in to your car via On-star without turning on the red “mic-on” light ,, this does not use the GPS portion of the system but rather it’s cellular function) ..
GPS trackers cannot be effectively stopped as they are hard to detect (receive only) and to hinder their operation you would need to interfere with the GPS signal being received and there is probably a law against that.. as the previous post points out they need to be hidden yet receive signal ,, they are often hidden under the plastic bumper covers as that doesn’t block signal ... you could drive a truck with metal bumpers and no simple place to hide a tracker but I can’t say that would be fully effective...
This is true of any GPS reciever - if it cannot "see" the sky (and satellites), it cannot know your location. As Neidermeyer ("Thank you sir, may I have another?") says, all you need to do is disconnect the GPS antenna or otherwise block it from viewing the sky. Take an ordinary hand-held GPS receiver indoors, away from a window. Or wrap it with aluminum foil. See how it works when the signal is blocked.
As for the GPS function in cell phones, I think you'd have to be a little more innovative. As others say, removing the battery in the phone will stop it from receiving information from the GPS satellites, and will prevent it from broadcasting that information. It will also prevent it from being a functional phone. I don't know where the GPS antenna is located in a typical cell phone, but if there's a way to block it, that would be good. Of course, it may share the same antenna that the phone uses to communicate, so blocking that antenna would render the phone useless.
Also note that even if GPS isn't available, the phone's position can be roughly located (very roughly, this isn't real accurate) through triangulation via the cell towers (again, removing the battery defeats this capability). If 3 towers can "see" your phone, the phone company can estimate your position without the help of GPS. It will be much less accurate, but will give a general idea, probably within 100-200 feet in a "tower-rich" environment.
How can one remove the plastic bumper cover to do a thorough search for these things? I realize they are probably installed just by reaching up and slapping them on where they adhere by magnetism, but...
Theoretically, there are ways of detecting electronic devices planted in your environment, even when they are turned off, but I think it would be hard to make those methods work in a metal-rich environment like a car.