If you would like to close your bank account, here is a sample template you can use to write an authorization letter to close a bank account.
Subject: Authorization to Close Bank Account
This letter is a written authorization signed by me requesting you to close my bank account at your bank named [Bank Name]. I need to close this bank account because [Mention reasons]. Fund transfer is not required, as I have already withdrawn all the amount from the account.
I am enclosing the bank account closure form and my identity proof for further process. I request you to kindly speed up the closing process. I will be highly obliged to you for this.
In case you have any doubt regarding this authorization letter, kindly reach me at [Email/contact number]