Still being charged after cancelling

I cancelled my LTE contract (due to pathetic signal) around 11 weeks ago. Two sets of cancellation forms were submitted (person at Stadium on main branch was not sure which one to use) via two separate fax numbers&two separate email addresses stated on the separate forms. I hand delivered both sets of cancellation forms to Stadium on Main branch on 7th of May (I have written acknowledgment of receipt of these forms from an employee). A month later I returned all devices and SIM cards in person to the Stadium on main branch in good working order (have signed acknowledgment of receipt of this) this was done to account for the one month cancellation notice period. The last two instalments after cancelling the contract I paid by credit card. Yet I have received a 'Letter of demand for R210' related to my bank not allowing debit order collection to take place-which I have now paid. I also received SMS's from a debt collection agency who advised I owe an amount of R1 371.64 which includes for service delivery in August. HOW CAN I BE BILLED FOR A CANCELLED CONTRACT? I do not buckle to ******ion-should Telkom blacklist me(as threatened in letter of demand) I will take legal action