Cancelling contract; again and again to no avail

In January 2016 I went to Cell C in Sandton City to find out about my existing contract, they informed me to return in Feb. My debit order went through 20th Feb'16, I went back just after that to cancel my contract. I was assured that the contract was cancelled and that the debit order would not got through. I duly cancelled the debit order with my bank too so it wouldn't go through on the 20th March. Cell C then debited my account on the 1st April. I went to see them immediately and was told by an Indian lady that they could not cancel it. After insisting and arguing for a long time they then found a cancellation form which I signed and they assured me that the amount would be refunded within 30 days. This morning (18th April'16) I received a call from the call centre asking if I want to upgrade my account! I then found out the account had ONCE AGAIN not been cancelled and that a further debit order was due to go off my account on the 20 April'16 and that the account was still fully active. I AGAIN returned to cancel it!!! What now? Who do I contact to confirm that is done ONCE AND FOR ALL?!! AND WHEN DO I GET MY REFUND/S?!!!