The easiest way to do this is usually to unplug the antenna and replace it with an RF terminator. The vehicle will simply think it is always out of range of cellular service. Unfortunately most modern vehicles require a lot of finagling to disable the telematics control module itself without causing error messages.

It would be nice if some regulator would mandate an "easy-off" function for vehicle telematics - some kind of simple procedure which would remove a telematics module from the installation list and allow the module to be unplugged without triggering fault detection. This is possible on some cars using dealership tools to re-train (sometimes called "code") the configuration blobs in each control module to omit telematics, but it's not standardized and usually too difficult for a consumer to manage.