Virgin Mobile South Africa
Contract ended in August, but still being billed

My name is Mortimer Petersen, Acc. no. 1090931594.
My contract ended in August, but despite my timeously informing the company that I do not wish to continue the service, and despite my informing the staff in Canal Walk on 5 occations, i am still being billed. This is turning into the account from, as the bank will not return the debits as I need a letter of authorisation from the supplier, and despite my emailing them on 2 previous occations, from which I had no response, They continue to bill me for a service I do not want.
Please Help.