If you bestowed specific authority to act on your behalf to someone in a special power of attorney form, and you no longer want that person to hold those powers, you must take those powers away. The process is called revocation. To revoke a power of attorney, a new document must be written and signed by you that makes it clear you no longer want your agent to act on your behalf.
Title your document "Revocation of Special Power of Attorney." Write this at the center of the page and use larger text; consider using bold print to make the intent of the document unmistakable.
Write "On [date], I [Your name], bestowed authority onto [your agent's name] in a document titled 'Special Power of Attorney (or whichever title).' I hereby revoke the powers given to [agent's name]."
List an effective date for the revocation. Write "As of [date of revocation], [agent's name] is no longer my agent and no longer has any authority to act on my behalf."
Take the document to a notary public and sign it in the notary's presence. Notarize the document. Deliver the document to your agent and deliver copies of the document to people you know have dealt with your agent. Consider publishing the notice in a newspaper in your area; do this if your agent did considerable work for you and you are not sure how many people the agent worked with.