MTN Debit Orders and Threats

I have a problem with MTN Contract. When i take the contract, i requested that they debit my account on the 25th of each month but I keep on incurring bank charges because they debit whenever they want on the 20 and 21 and when they don't get the money, they always threaten to block my number. I tried to call them but whoever I spoke to kept on telling me they will change it on the system but nothing is happening...this is now 5 months and I have been incurring the charges all these months. Can someone please tell MTN to stop debiting my account, I will pay manually (as I have been doing anyway) because when I try to make things easier by setting up the debit order, its like now I gave them permission to do as they wish with my bank account.