If you would like to revoke an authorization you granted someone earlier, here is a sample template you can use to write a revoke authorization letter.
Subject: Revocation of Authority
I, (Your Name) the undersigned, revoke the authorization of (Authorized Name) to take actions in any matter on my behalf. The revocation of authorization is effective from (Date). I request you to relay the information to your department for future references.
Thank you for your co-operation. Please let me know if you have any questions.
(Your Name and Signature)