You'd best do it from the Sprint side of things (data block); every time I see a complaint about massive overage charges it seems the poor person "tried to block data on the phone" but something ended up connecting in the background when they were traveling in outer Siberia and the roaming charges required taking out a home equity line of credit just to cover the interest.
I will pre-echo the common sentiment that to have a TP, the highest-end WM smartphone in Sprint's lineup, without a data plan is sort of like buying a car and taking the wheels off. Yes, you still have a car, but what's the point.
I know, you said you never use it...surprising to me, but whatever works!