I'm curious - what happens to these cars (not specifically GM's, just these "smartcars" in general) when you disable/disconnect/destroy the (presumably) SIM card or whatever it uses to connect to cell data?
OnStar's module is in deeply embedded inside the car and in different locations in different models. OnStar is tied into the vehicle diagnostics and electrical system. If you manage to find it, and pull its cables out or something straightforward like that, your car will probably report an engine error and not start. (This issue has been reported and discussed extensively in car hacking forums.)
I asked my local GM dealer--a very large dealer, by the way--about disabling OnStar permanently (at the hardware level). They told me (a) they don't know how to do it, (b) I'm the first person to ever ask about it, (c) they think it might void the warranty (I don't know if they are right or wrong), and (d) they're unwilling to do it.
[1] Edit: Which is something I wanted to use in the sentence, but now I see I didn't. This is not a good day for me.
Sadly, I'm not even being sarcastic.
GM dealers are all supposed to know how to disable OnStar, and indeed it is usually actually quite easy for an end user to do, with no negative impact on the vehicle. I have a 2010 Traverse and disconnected the OnStar module and antennas with no negative impact outside of OnStar -- it is in an easily accessed compartment near the back of the vehicle.
Some dealerships simply never deal with this, though, just as they are supposed to know how to disable the passenger side airbag but many have no clue and act incredulous. It just isn't that common.
GM doesn't widely share the information on disabling it because ostensibly a purpose of the system is theft recovery -- that if your car is stolen they can track it, which becomes less achievable if every thief just pulls a fuse or something. Nonetheless the information is out there and easy to find.