Settle account but still being harassed

The most useless group ever do not get finance from them ever. Paid up account but forgot to ask for a settlement letter. The had my bank details and permission to debit my account bit did not. Putting a non payment on my credit rating for a month. Then requested settlement confirmed before my account was debited with the amount that all was in order. They debit the money form my account. They have been harassing me ever day atleast 6 calls a day with me telling them this is settled but they will say will excallete etc. Then I get another call. Then get told as I did not get the settlement letter sent to me it is not valid??? Request letter then so I can settle and confirmed my email. To date nothing.