Contract termination loophole

I’ve left Germany about 13 months ago and I need help dealing with O2’s terrible business practices.

After going through the borderline unethical contract termination procedure and failing to get O2 to cancel my contract I gave up and paid the remaining 12 months left in my contract. I paid the (supposedly) final bill happy that I would never have to deal with O2 again.

I was surprised a couple weeks later when €4 bill arrived in my email. I had already given hope and assumed this was just some last attempt to squeeze whatever money they could from me. I paid the bill assuming this was it.

Four weeks later I get another €4 bill. This time I check the bill and learn that they were charging me a fee for attempting to get a direct debit from my old bank account. This account was closed 18 months ago and I informed O2 of this when I was trying to terminate my contract! Apparently they never stopped trying to debit from this account so I was paying this €4 fee every month without noticing it. Now it seems they are charging me a €4 fee for failing to debit the previous €4 fee. I go to the website and learn there is no way to communicate this problem or update the information so I got to their online chat support service. I explain the situation and the person in the support chat tells me that the problem will be fixed if I pay the bill. Naively I comply as I had already prepared the transfer and I was desperate to be rid of this issue. 

Now I get another €4 bill but now I can’t even contact them as my o2 account was closed ( since my contract was actually terminated). So what is O2’s plan? Charge me €4 every month for the rest of my life?

I don’t know whether the cause of this is dishonesty or stupidity but I know I will make sure to advise anyone I know to keep their distance from this company.