Power of attorney is a legal document, which gives power to the attorney holder. By giving Power of Attorney to someone is gives them the right/power to carry out certain specific financial and legal businesses on their behalf. The relationship between the two parties is similar to the relationship of Principal and agent. It is common among NRIs, for example, to give Power Of Attorney to someone they can trust and to represent them to carry out their businesses in India since it is difficult for them to travel frequently in the country of their origin.
Now if you have registered the GPA, it can be cancelled easily. Power of attorney is said to be Revocable If it is revocable at the will of the principal. And Irrevocable If it is not revocable at the will of the principal. If you have mentioned a clause in the POA document then, you will have to abide by it. You can revoke the GPA by sending a notice to the GPA holder and by filing a revocation deed. Revocation deed is a written legal document that states the intent to revoke the previous deed i.e. Power of Attorney.
With this deed, the power to make a decision with the agent is nullified and it gives back the power to the principal to make any decision regarding the subject matter of POA. Also publish the cancellation deed in the local newspapers. Also inform your father about the same.
Reference: Transfer of Property Act, 1882, and Registration Act, 1908