You can contact your agent by phone and revoke her authority orally first. Placing the revocation in writing and sending the document to her by mail with a return receipt gives you proof the agent knows about the revocation. You may draft the revocation yourself or use a form from an office supply store for use in Ohio. All revocations must identify the power of attorney itself -- such as by date executed and the parties involved -- and state the revocation of the agent's authority. While you don't have to have your signature notarized, doing so will eliminate any doubt about your signature's validity.
You must send a copy of the revocation to any third party, such as a bank, you filed the power of attorney with. You may send a copy of the revocation by mail to any third parties with the power of attorney on file, but you may need to visit the locations personally if you need the revocation recognized immediately. If your agent uses her authority after you're revoked it, but with a third party who didn't receive any notice of the revocation, the third party isn't liable to you for any resulting losses.
If you don't notify your agent when you revoke her authority, she's not liable to you for any damages arising from actions on your behalf as long as her actions were in good faith and honest. Any actions she took in good faith, without knowledge of your revocation, are binding. For example, if she signed loan documents for you, you're bound to the loan agreement.
If you sold, bought or mortgaged real estate in an Ohio county, your power of attorney was filed in that county's land records at the same time as the documents from the real estate transaction. You must file your revocation in the land records of each Ohio county in which you used the power of attorney for real estate. Ohio laws recognize the revocation of a power of attorney in the land records only if the revocation is also filed in the same land records.