Not cancelling contract after many requests

I have a data contract with Telkom Mobile which I have been trying to cancel since April 2015. I went to the Canal walk branch and from there was told that I needed to give 30 days notice so I would be billed one last time. I was told that I would receive a call from someone to confirm. I never received that call. I got emailed asking for a number, but no one ever called me once I provided it. I was billed at the start of May due to the 30 day notice, but when June came around I was billed again. I went back to the store and was told that allegedly someone had tried to call me, but they weren't able to reach me so they decided to not cancel the contract. After ensuring that all my contact details were correct I asked them to relaunch the process. I again received no calls and was again billed at the start of July. I then called the call center and was assured by the agent that my account was indeed now cancelled and that I would not be billed anymore. When the start of August came around I was billed once again. I'm really tired of this, I would like this contract cancelled immediately and would like to be refunded for this error. Three months worth of payments by my count.