[THunDA]

You could always delete the connections at..
Start>Settings>Connections>Connections>Manage Existing Connections

Only problem is I would think you'd still get an error from things trying to connect and finding no connection... but it would ensure the phone isnt connecting.

You might also be able to call sprint and having them block data on your phone, im pretty sure this can be done.

Although im sure someone else will come along with a better way of doing it.