That'd be tricky. You could stop the device talking pre-LTE/4G and UMTS/3G and rely on 2G which will be more than enough for texts and old fashioned mobile calls (the new hifi calls rely on 3+G I think, but I've never knowingly made one of those myself yet).
But even if you only have the 2G modem on it'll still have to ping towers to advertise which towers if can see so the network can route incoming calls to your pyra. So even if you shut off mobile data, the owners of the towers can still track your location, and as we've seen then be compelled to tell various levels of government about you whether they care or not. If you don't want them to track you you're going to have to turn that chip off entirely (which I believe is possible, though I've not checked the schematics recently), and be very careful about where you connect to wifi. In practice I believe MITMing and dodgy javascript are more of a threat to my private data and personal liberty than the government is who just know where my meatspace body is, by the way.
In practice though, I suspect downing whichever network interface is attached to the data modem is probably the simplest way of forbidding mobile data. The chip can then connect over whatever radio encodings it likes, but can't inject IP packets into the system's network. It could self originate packets possibly, and this may happen if it decides to download an incoming MMS (picture message), or it initiates a hi-fi audio call (if it even supports that natively), but both of these probably come out of your calltime/text budget anyway, assuming it's the money that you actually care about.
