In the case of a Pope, the warning is not an act of jurisdiction, but an obligation of charity (fraternal correction), as Bellarmine explains
Since the council and Benedict's obedience agreed that it was necessary for him to be legally deposed, and since the council had the authority to do what was necessary to end the schism, the Council of Constance did not err when it legally establish Benedict's crimes, declared him deprived of any title, and deposed him. If it did err in doing so, then Pope Martin V erred in approving it.
“there exists no possibility for canonical warnings to be administered to a pope” (Kramer).
In the case of a Pope, the warning is not an act of jurisdiction, but an obligation of charity (fraternal correction), as Bellarmine explains:
“Peter allowed himself to be reprimanded by Paul because that was not a juridical censure, but a fraternal correction. For, as Augustine says in letter 19 to Jerome, and Gregory in homily 18 on Ezekiel, Paul does not reprimand Peter, as a superior judges inferiors with authority, but as inferiors sometimes correct their superiors out of charity.” (De Romano Pontifice, lib. 2, cap. xxvii).
Now read again what the Council said concerning Benedict XIII:
Council of Constance: Benedict XIII … has sinned against God’s church and the entire Christian people, fostering, nourishing and continuing the schism and division of God’s church. How ardent and frequent have been the devout and humble prayers, exhortations and requests of kings, princes and prelates with which he has been warned in charity, in accordance with the teaching of the gospel, to bring peace to the church, to heal its wounds and to reconstitute its divided parts into one structure and one body, as he had sworn to do, and as for a long time it was within his power to do! He was unwilling, however, to listen to their charitable admonitions.”
“a pope cannot be judged for so long as he is pope, because it pertains to the nature of judgment that it is an act of jurisdiction.” (Kramer)”
A pope cannot be judged with a coercive judgment while he remains pope, but he can be judged with a discretionary judgment, as Bellarmine and everyone else admits. A discretionary judgment is a legal judgement, but one that lacks any coercive force. The nature of the judgment is merely to decide or determine facts, not to punish or impose penalties. Bellarmine explains that a discretionary judgment is the form of judgment used by an Arbitrator, not a judge in the true sense of the word. He also lists many Popes who were indeed judged in this manner, including Leo IV who willingly submitted to the judgment of the Emperor and agreed to obey whatever was decided.
Since a discretionary judgment does not include any coercive force it is not, of its nature, forbidden in the case of a Pope. This is the form of judgment the Church would exercise in the case of a Pope who was accused of heresy. The Church would investigate charges and reach a verdict (discretionary judgment); Christ would authoritatively depose the Pope by severing the bond that unites the man to the office; then the Church would judge and punish the former pope (coercive judgment). As we will see later, this is how Bellarmine himself says the process would unfold, in quotations that you will never find on a Sedevacantist website.