There may be a fuse you can remove.

Move to the Yukon

A lot of cars completely disable the modem when service expires (which makes sense since they don't want to pay $5/month to keep it active) which makes your car harder to exploit. But if you want more assurance, it takes less than an hour to unplug the antenna in your car. Look for leaked service manuals, repair videos on YouTube, or search for a replacement module so you can look at diagrams on parts websites.

Once you know where it is, you can pry the interior panels and find the module (it should have an IMEI on it). Then find the cellular connector (it's often a standalone coaxial antenna/cable).

For example, my Hyundai has cellular connectivity integrated into the head unit. I removed 2 screws to pry off the dashboard trim, removed 2 screws and 2 bolts, and pulled out the head unit (10 minutes). There were multiple coax connectors (such as for the FM antenna and satellite radio), but the cellular module was clearly visible through the ventilation slats so I yanked its connector (5 minutes). I taped the connector to the wiring harness so it doesn't rattle and reassembled everything (10 minutes). Less than 30 minutes overall, not bad for a hardware mod!

I'm not against these systems because they offer a lot of convenience for people who need them. Some people need to pre-cool/pre-heat their car when it's parked several blocks away from their home or office, or they need warnings if they absentmindedly leave a window open or a door unlocked. Not to mention you could get to your car quickly and deter a thief if it sends glass break notifications. That's not worth the software vulnerabilities nor $10/month to me, so I disconnect the antenna for peace of mind. But some of my family members are glad to use these features because it improves their QoL. So I'm in favor of legislation that mandates a few years of security updates, and requires that a disable switch will actually disconnect the modem.