the only thing that I can think of that could prevent the entire population of hell from repenting is by the taking away of their free will
but I don't see how you could reconcile that with the character of God ???
Once one dies in their sins, the offer of salvation is null and void. Salvation is a faith issue. Who in their right mind would not except the offer of salvation after they find out the truth and have gone down into Hades to begin their punishment?
Jesus gave us the answer to the question of whether a person can be saved after death as found in the event of the rich man and Lazarus.
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Then I beg you, father,’ he said, ‘send Lazarus to my father’s house,
28for I have five brothers. Let him warn them, so that they will not also end up in this place of torment.’
But Abraham replied, ‘They have Moses and the prophets; let your brothers listen to them.’
‘No, father Abraham,’ he said, ‘but if someone is sent to them from the dead, they will repent.’
Then Abraham said to him, ‘If they do not listen to Moses and the prophets, they will not be persuaded even if someone rises from the dead.’ ”
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No one can come back from the dead to warn the living. However, what Abraham said to the rich man was that, his brothers can find the warnings to keep from going to Sheol/Hades, by reading the words of Moses and the prophets, i.e. the word of God. But once a person dies, he cannot come back and warn anyone.
Today (while you're alive) is the day of salvation! After you die, your record is sealed and the offer is off the table.