ADT Finance/Accounts Department is a nightmare

I cancelled by contract with ADT, they gave me outstanding fees i need to pay, I paid all outstanding fees and send them proof of payment. Since June i have been asking for a Letter indicating that the account is paid up and closed, the Lady by the name Marry Cherry from Finance department was handling my account. She's terrible to deal with, i sent her numerous email without any single reply, called the Call center million times and still no help. I just need a letter to sent to Transunion that the ADT account is closed and no longer active.