I need anunauthorised debit order to be stopped

I went to the bank after I noticed an amount of R99.99 has been debited from my account for more than 4 months so I asked that they cancel it as I don't know who those people are. They said firstly they will charge me R45 to stop it but it will not be a final solution as these people might change the amount/reference and still get my money. They then also say that I gave these people permission maybe over the phone or Internet to debit my account of which I've never done. I need these people to help me stop this but they give me problems instead of solutions. I need this thing cancelled all together please help and this bank isn't helping me with nothing. How do they allow anyone to just come and debit my account? How do these things work? Because I'm of the impression that a third party must produce proof of our agreement before my bank can allow them to debit my account but the bank has told me otherwise. This means my money is out in the open and unsafe as the bank doesn't care who takes it and who doesn't.