telkom thieving

I have cancelled my mobile service way before the expiry date, yet even after i have contacted Telkom numerous times they still insist on debiting my account. The sim has been blocked since last year October, the contract finished in January, yet I am still expected to pay. I have the proof of cancellation, I spoke to a Tangag on 8 March, he assured me this issue is now resolved and yet they still trying to debit my account. I am totally at a loss as to where to now. I am being charged for a service I do not have