Telekom does not cancel my account

I have canceled my Telekom line and account by giving them 1 months notice and paid for the last month notice for February 2020, but they keep on deducting from my Bank account, calling their customers service every month only to get a reference number and nothing further happens, all I can do is to reverse the debit order every month, in the end I will end up getting a bad credit score, please cancel my account