Unauthorised debit by MTN

I had a data contract with MTN for some years. When the contract expired earlier in 2021 (I don't remember the exact month) I did not upgrade it because I don't need that data anymore. I then called them to cancel the contract but up to date they still debit from my account despite the fact that the contract was cancelled. Every time they debit I reverse the pay as unauthorised debit but the following month they still debit. Sometimes they call me telephonically to enquire about my non-payment, and I explain to them that I cancelled the contract, nevertheless they still debit for the cancelled contract. I am not going to pay for the cotract that I canelled a long time ago.