Unauthorisied account debits

Same problem as the previous one. I receive an sms today after 12am to advise that my account will be debited with a certain amount and another sms within a few minutes advising another amount. I pay my monthly instalment. No one called or sent an email or message to make this arrangement with me. African Bank does as they they please with my bank account- debit any amount they feel is convinient for them. Is this fair to us as customers. I called and spoke to a consultant who told me i won't be getting my money back as it is already debited. I asked to speak to the manager and was advised they will be called within an hour and since then, nothing. Just an sms thanking me for paying my account. I am very dissapointed and angry as i have a monthly budget and for money to be debited without me authorising that, it is not right and ********.