Poor contract service

I took a 24 months SIM only contract with MTN in 2020, and everything went well. When it expired last year, 2022, I didn’t renew it or upgrade to a new contract, with the hope that my number will revert to prepaid. Instead, they started providing me with 6GB per month and deducting it’s payment, monthly, from my bank account, all without my consent. When I visited two of their different branches to cancel that, they sent me from pillar to post, until, finally, I called their service centre. I was then sent a form to cancel the contract and revert my number to prepaid. I filled the form and emailed it back. That was the last I heard from them. They continue to give me 6GB power month and deduct from my bank account. So I finally resolved to reverse their deductions and move my money out of the bank account so that they will find nothing.