Agreed with the comment below, the ball is firmly in your mobile provider's court. The contract will almost certainly state that service can be withdrawn at any time on their sole discretion for any suspected breaches; if you're tethering on a contract that doesn't allow it, they're well within their rights (which you've agreed to by accepting the contract) to block your service until you comply or close your account completely. You have no recourse in this situation according to their T&Cs.

They don't need to prove it and they have no obligation to give you your money back, either. Be careful.