Still paying for a Cancelled contract

I cancelled my contract 6 June by phone and by mail. Since then 2 debit orders went through and the next one for August is scheduled. After 5 calls to them, and many hours on hold I was told that they can't do anything about the debit order, it must go through and then afterwards I can phone them and start with the claiming back of my money. If I try to stop the debit order then they will but me on the credit bureau as an overdue customer.