MTN not letting me cancel my contract with them

I have been calling to cancel my contract with MTN which ended in December 2017, they keep putting me on hold for more than an hour. When I go to the store they tell me I can only cancel by calling 808. They still deduct money from my account every month and I can't stop the debit orders because they'll hand me over. They gave me a reference number : ********** 00. But nobody from MTN has called me back. Ironically agents from upgrading are always available, the minute I tell them I want to cancel they transfer me to another department which just puts me on hold. I don't know what to do anymore.