Send a copy of the Revocation of Power of Attorney to your attorney to let them know that you no longer want them to act as your attorney.
If you post the Revocation of Power of Attorney to your attorney, consider using recorded delivery in order to have proof of delivery.
If you hand-deliver the Revocation of Power of Attorney to your attorney (eg post it directly into their letterbox or personally give it to the attorney), consider having a witness with you who can attest to the delivery of the Revocation.
If your bank or other financial institution has a copy of the power of attorney, let them have a certified copy of the Revocation so that they know not to deal with your attorney any longer.