Debit order dispute

I called the call centre to dispute a debit order and I was told that there is a mandate and they cannot dispute the debit order, however I disputed it in December already as my contract with them was terminated by me, the agent who was assisting me was very rude and inaudible by the name of Lunga which I was given by a female manager since it seemed like he did not want to talk to me and failed to explain what is a mandate, I feel they took me like I am nothing because I use a savings account and I am not a private banker.