Failed to cancel contract since last year

I won't recommend this company at all, worse services since last year, I took a contract with MTN/Mondo, I wasn't happy with the headset and return it within 3 days. MTN accepted the headset and Mondo did acknowledge when i called them that they received the headset.

They promised to cancel the contract within 30 days which they have failed and this has been going on since last year. They have been debiting my bank and if I reverse the debit order the update on my credit profile as a bad payer.

Their caller agents have been promising that the contract will be canceled as it was supposed to and to date they haven't and now messing with my credit profile if I don't pay