The Ben Yehoyada provides two understandings:
- He explains that generally gehinnom acts as a tikun, a rectification for the judgement of the rest of the world but not so for the judgement of reshaim (evildoers) as they have their own portion in it, as per the Gemara in Chagigah 15a:
אָמַר לוֹ רַבִּי עֲקִיבָא רַבְּךָ לֹא אָמַר כָּךְ אֶלָּא בָּרָא צַדִּיקִים בָּרָא רְשָׁעִים בָּרָא גַּן עֵדֶן בָּרָא גֵּיהִנָּם כׇּל אֶחָד וְאֶחָד יֵשׁ לוֹ שְׁנֵי חֲלָקִים אֶחָד בְּגַן עֵדֶן וְאֶחָד בְּגֵיהִנָּם זָכָה צַדִּיק נָטַל חֶלְקוֹ וְחֵלֶק חֲבֵרוֹ בְּגַן עֵדֶן נִתְחַיֵּיב רָשָׁע נָטַל חֶלְקוֹ וְחֵלֶק חֲבֵרוֹ בְּגֵיהִנָּם
Acḥer said to him: Rabbi Akiva, your teacher, did not say so, but explained the verse as follows: Everything has its opposite: He created the righteous, He created the wicked; He created the Garden of Eden, He created Gehenna. Each and every person has two portions, one in the Garden of Eden and one in Gehenna. If he merits it, by becoming righteous, he takes his portion and the portion of his wicked colleague in the Garden of Eden; if he is found culpable by becoming wicked, he takes his portion and the portion of his colleague in Gehenna. (Sefaria translation & notation)
- Alternatively, the Ben Yehoyada reasons that Gehinnom is made according for everyone according to the value of their deeds and their resulting punishment. And this 'burning' (i.e. punishment) ends and renews a person, and this is, as it were, their gehinnom. So it is like a refinement process that a person goes through in order to come out 'purer' and free of sin. This is what it means when it says that Gehinnom will end but they will not end - i.e. the process will initiate the 'reset' button but then as reshaim they will need to experience a new process of gehinnom. So the process ends but they will not and will have to go through it afresh.