Please stop debiting my account!!

I have been experiencing problems with CELL C for the past 4 months. It has become extremely frustrating and inconvenient as I have requested them to cancel my debit order and notified them I will make a monthly cash payment due to the fact they decide to debit my account whenever they feel like for whatever amount they please and I have to go to the bank and return the debit order. I have sent numerous emails and called through to their call centre and not one person can actually get a simple job done. I cannot understand how a service provider can have so many staff and none of them are actually willing to do their jobs.