A Power of Attorney deed is always registered and made legal through proper stamp duty and registration process. Hence as long as the deed remains in the hands of the Attorney he is legally entitled to enjoy the powers granted in the deed. And the principal cannot cancel it orally. Cancellation of Power of Attorney is also known as Revocation of Power of Attorney in the legal language.
To revoke or cancel the Power of Attorney granted by the Principal and if registered, the same Principal (the person who granted the power) has to draft yet another Revocation Deed expressing his intention to cancel the earlier deed with effect from a particular date.
Revocation Deed is to be drafted in a proper format on a stamp paper of due value and has to be presented to the Registrar or sub- Registrar office where the earlier deed was registered. The Revocation Deed also has to be registered if the earlier Power of Attorney deed was registered.
You also have to send a written notice to the person to whom the power was granted, that is the Attorney, mentioning the cancellation of the deed from the date of the notice. Otherwise, if the Attorney comes to know of your intention before the notice is given to him, he may opt to do some mischief in case he has any crooked ideas.
If the Power of Attorney is not registered like the case of Special Power of Attorney, where it is just made on plain paper, and notarised, then a written notice to the Attorney mentioning the cancellation is enough.