Cancelled contract but deductions are still made from my account

I have cancelled my cell phone contact in may 2017 and deductions are still being made from my bank account. I visited a MTN shop and they explained to me that they cannot assist as they not allowed to cancel contracts. The gave me a number to call. I phone the call center on numerous occasions and explain the problem but was every time transferred to other employees. I then send emails which was answer with 'call the call centre'. I done that again but operator requested I phone again because the system is frozen. It appears that it is impossible to cancel a debit order as I have tried the shop staff, emails and call centre.