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Absolutely. The Principal simply needs to execute a "revocation" of their POA and issue it to their agent (the person who formerly held the POA).
The Principal should also send this document to their financial institutions, and anyone else they do business with, to prevent them from transacting business with the POA.
Oregon Revised Statute: http://www.oregonlaws.org/ors/127.015
Add'l information: https://www.osbar.org/public/legalinfo/1122_PowerofAttorney.htm