DSL Contract termination failure

Hi O2 Help,

I signed a 2-year contract which began to be effective on August 2013 and my package costs 24,99 euros per month.
In May 2016 I asked O2 to cancel the contract, because my new apartment already had internet, in a week I received a phone call from O2 asking for my new registration details. During this phone call I explained that I have an appointment for a new registration in two months and O2 contact person told me that it's not a problem.

After that in august 2016 i sent to O2 my new registration details. However, I received a reply from O2 that my contract was renewed until August 2017.

So since June 2016 I pay to O2 for a service which i don't even receive.

Could you please tell me what should I do, and how to avoid this situation in future?

I think this topic would be interesting for a lot of people in Germany, because it's so easy to make a contract with O2, but apparently, it's impossible to cancel it.