Bad service

I phoned Finchoice to arrange payment they gave me the balance and I paid up and asked for paid up letter which was sent to me two months back by the lady called Lizzie.She sent me a paid up letter and I asked her to close the account.The account is still not closed instead they sent me a message claiming that i owe them when enquired they told me i had an account in arrears i told them about paid up letter and my arrangements then they started to tell me that i had another account that they have never told me about in the first and that i do not know of.I asked a call back no reply till today and account is still active on my credit report.