Non- authorised deductions

Our school was contacted, much the same as so many others on this page, on an incredibly bad line (I presume on purpose) and after what we thought was just confirmation of our current Telkom details have now been charged R792 monthly for a listing we DO NOT WANT and did not knowingly authorise. In fact could not authorise as the phone call was not taken by the account signatory. We have tried phoning and submitted an online query to yourselves and were assured on 8 June that your agents would be in touch . This has not happened. WHAT CAN WE DO TO STOP THIS ********** CHARGE BEING DEDUCTED FROM OUR TELKOM ACCOUNT AND THE AMOUNTS REPAID?