Yes, a power of attorney can be revoked at any time by the person who made it (assuming they have the legal capacity to do so). Revocations should be done in writing, and the revocation of the rights to make decisions should be adequately communicated to the person who previously held the power. Additionally, if the person who has made the power of attorney is no longer capable of revoking the powers, but there is evidence of malfeasance, a court of competent jurisdiction may suspend the powers as well.