Won't cancel service after repeated requests

Canceled my 4 contracts with rain on 16 July. To be told that they would only be canceled at the end of the month.

Com the 20th of the next month one on the 4 contracts is still on my account.... Suprise suprise it's the most expensive one. I cancel it again

This time I get a call from the retentions department. I ask why it was not canceled already they appologies for this and tell me that they will cancel with immediate effect. But I will still have to pay for another 18 days of usage... I begrudgingly settle for this.

Now I log on and see it is still on my account, what do I have to do to get this canceled? Change bank account?

Why the hell must i pay another 18days for there mess up?