Unauthorised Debit Orders to my Account

In March 15 I complained about the poor signals and no service from Cell C on HelloPeter. The next thing they cancelled my contract without any notice. I was happy at that stage as I was able to move to Vodacom immediately where I have had no issues since then. During my 2 year contract I never ever received any statements and that was one of my complaints. This was a data bundle contract of R99pm for my iPad. I did not ask them to cancel immediately. Now they try and debit my account twice per month with an amount of R624 which I have a permanent stop payment on. They refuse to provide any details what this is for and I refuse to pay this. Cell C you can forget to ever get this amount from me so stop debiting my account. Your absolute uselessness simply just continue to amaze me. To everyone out there.....do not fall for their latest gimick of promising you R10K to buy yourself out of your current contract and join Cell C....you will regret that for ever....