If he is right, the man on the chair, prior to the declarative sentence, is still a true pope. Impound him - whatever - he is still issuing, or has issued, false and/o evil teachings and laws while in the seat.
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No human being has any jurisdiction, whether spiritual or temporal, over a true pope. That means no one can pass sentence against a true pope.
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This is why the Fathers of the Church don't teach any such thing. They all teach (and St. Robert Bellarmine says it is the unanimous teaching of the Fathers) that a pope who taught heresy would fall from office by that very act, and the cardinals would then make a declaration of the fact that he had already lost the papacy and then proceed to elect a pope. But obviously this is only possible if the former-pope had already lost his office before the cardinals made a declaration to that effect and then elected a replacement.