Cell C refuses to cancel my account when the sim and device have been ****** and are being used by someone else.

I have a sim with cell c that only received data monthly. It is in a 3G modem. This was ****** from my house and someone is using it. I contacted cell c online and went into the store to stop and cancel the account. I was told I have to pay it as their account works in arrears, so I made the payment. The account has still not been cancelled, the other person is still using it and I'm being billed for it again this month. If cell c does call, it's their promotions department, cell c upgrade and they call when I'm teaching and cannot take the call. I want the account cancelled but I'm getting no assistance from cell c