Unable to cancel account

Last month on the 15th, I've given notice to cancel our business telephone line and ADSL (Basically everything except the SmartAccess package). I completed a cancellation form and provided a signed letter on a company letterhead and gave it to the Telkom Shop at Mall@Reds. On the 10th of this month, I received an invoice for up to 8 May which means the service has not yet been cancelled as requested. I tried using the communications options on the Telkom website (Email, Chat) but they all prove to be absolutely useless since there seems to be nobody monitoring those channels.