Change IBAN and name of contract

Good morning,

My name is Carina. I had DSL in my flat, but unfortunately I have moved out some months ago. Who is living in the flat and using the internet is a friend of mine. We already tried to change IBAN 2 or 3 times (for my friends bank account), but they still keep taking money from mine. Last month, in a desperation tactic, I asked the money back from the internet payment, hoping O2 would realize there is another IBAN associated. Yesterday, my friend called me saying there is no internet anymore. How can I get rid of this mess situation? All I want, is change the IBAN and name! Help please