Not happy because I have been trying to cancel my month to month contract and I was advised it's not possible because I have an amount that is in arrears

Not happy because I have been trying to cancel my month to month contract and I was advised it's not possible because I have an amount that is in arrears. However I have made an arrangement to settle the arrears amount in cash on 30 June 2021 and asked for debit order to be canceled because it will bounce DUE to no funds in my account also I was told I can not cancel the debit order and my concern is that I get charged by my bank and cell c for a failed debit order. I just need to cancel my contract and settle the outstanding amount.