To revoke a prior tax information authorization, the taxpayer must submit a new Form 8821, Tax Information Authorization, with a clear indication that the previous authorization is being revoked. The new form should be sent to the IRS as soon as possible, and the IRS will process the revocation as soon as it is received. It is important to note that revoking a tax information authorization does not affect the validity of any tax information that has already been disclosed to the designated person or entity before the revocation was processed.