Poor service

I paid off my loan in March and got confirmation from the Wonga agent I spoke to telephonically. I got an email confirming that the account was fully paid and asked for a paid up letter. I then received a paid up letter with incorrect ID number on which I immediately queried with the company. A week later, I got numerous calls from their attorneys threating to take legal action as my account is not paid. The statement on my Wonga account shows that it is fully paid. Despite following up with Wong numerous times and advising them that my account is paid, I am still getting threat messages and Wonga is ignoring my emails.