They refuse to cancel my service and keep debiting my account

I cancelled my service and paid a full calendar month that I didn't even use plus a 'collection fee'. However, they still went ahead and debited my account the next month. I just can't get rid of this ******* company. Now I have to arrange with my bank to cancel the debit order. Will Supersonic make it count against my credit record? Probably. Then I'll have to fight them on that. Why can't I just get rid of this ******* company???