Poor Sevice and False Accusing

My account was handed over to debt collector 'cause it was in arrears one month. I paid the amount in full. Thereafter I requested Mr.P to close my account as I do not want an account anymore. After it was paid, the debt collectors kept on threatening me and said there is more money outstanding....for interest !! Interest on what ?? I did not buy anything on my account after it is fully paid. I phoned the accounts department at Mr. P and they said they cant do anything about it as it is at the debt collectors. I have emailed the proof of all payments done to the debt collectors and now I still keep on receiving threatening messages. I am so fed up with Mr. P and their bad service. I will not recommend anyone to ever open an account with Mr.P !! They want your money every month but when you are paid up to date then they want interest just because they can !