UNAUTHORISED Debit off my bank account

I logged a request to cancel my contract a few months ago. I received an email stating I will have to pay in R6700. So I did not respond to the email, as it was too much to pay in and I could not afford it. This morning, a DEBIT order went off my bank account for R6700, for Telkom. HOW was this allowed?? I did not give consent ANYWHERE for a debit amount to be taken off my account other than the current monthly installments. Telkom does not have ANY documents where I gave consent to go ahead with the cancellation of the service as well as the SIGNED document where I confirmed they can debit off the cancellation fee from my bank account. I WANT MY MONEY BACK. This is unacceptable. I have other accounts to pay, now I am going to occur bank charges for insufficient funds because my other debits won't be able to go off. WHO AUTHORISED the debit to go off my bank account????????? How was this allowed by FNB? Surely a check should be done to confirm that kind of amount to be debited from my account???