Refusing to close prescribed account

The service is terrible and it's affecting my life so bad. They have been calling me for months regarding an account that has been prescribed. I have sent them the letter numerous times but nobody is assisting in closing the account. They have gone as far as to hand the account over to Ver-tex solutions and I also sent the the prescription letter. I feel soo helpless. I don't know what to do anymore. This is soo frustrating