R' Yossi Mizrachi mentions in a YouTube video that the Rambam (Maimonides) writes in Hilchos Teshuvah:
The biggest punishment to a Jew is God closing the gate of teshuvah (repentance) in his face.
Does anyone happen to know where in Hilchos Teshuvah that this statement is located?
In order to find the passage you're seeking, I did a Google search for [ Rambam | Maimonides punishment God closes the gate of teshuvah ]. Baruch Hashem, I found something.
The first bit of Hilchos Teshuvah 4:2 begins:
ומהן חמשה דברים הנועלים דרכי התשובה בפני עושיהן ואלו הן
Dr. Henry Abramson translates:
And among them are five things that close the doors to the ways of teshuvah, and these are they
Rabbi Eliyahu Touger translates similarly:
Among [the 24] are five deeds which cause the paths of Teshuvah to be locked before those who commit them. They are:
You may view the full Hebrew text of the chapter, plus Rabbi Touger's translation, on Chabad's website.
Section 4:6, at the very end of the chapter, adds:
All of the above, and other similar transgressions, though they hold back repentance, they do not prevent it entirely. Should one of these people repent, he is a Baal-Teshuvah and has a portion in the world to come.
I believe the reference is probably to this passage (Laws of Repentance 6:3), in which he explains that a person can sin so badly that his free will can be taken from him:
וְאֶפְשָׁר שֶׁיֶּחְטָא אָדָם חֵטְא גָּדוֹל אוֹ חֲטָאִים רַבִּים עַד שֶׁיִּתֵּן הַדִּין לִפְנֵי דַּיַן הָאֱמֶת שֶׁיְּהֵא הַפֵּרָעוֹן מִזֶּה הַחוֹטֵא עַל חֲטָאִים אֵלּוּ שֶׁעָשָׂה בִּרְצוֹנוֹ וּמִדַּעְתּוֹ שֶׁמּוֹנְעִין מִמֶּנּוּ הַתְּשׁוּבָה וְאֵין מַנִּיחִין לוֹ רְשׁוּת לָשׁוּב מֵרִשְׁעוֹ כְּדֵי שֶׁיָּמוּת וְיֹאבַד בְּחֶטְאוֹ שֶׁעָשָׂה. הוּא שֶׁהַקָּדוֹשׁ בָּרוּךְ הוּא אָמַר עַל יְדֵי יְשַׁעְיָהוּ (ישעיה ו י) "הַשְׁמֵן לֵב הָעָם הַזֶּה" וְגוֹ'. וְכֵן הוּא אוֹמֵר (דה"ב לו טז) "וַיִּהְיוּ מַלְעִבִים בְּמַלְאֲכֵי הָאֱלֹהִים וּבוֹזִים דְּבָרָיו וּמִתַּעְתְּעִים בִּנְבִאָיו עַד עֲלוֹת חֲמַת ה' בְּעַמּוֹ עַד לְאֵין מַרְפֵּא". כְּלוֹמַר חָטְאוּ בִּרְצוֹנָם וְהִרְבּוּ לִפְשֹׁעַ עַד שֶׁנִּתְחַיְּבוּ לִמְנֹעַ מֵהֶן הַתְּשׁוּבָה שֶׁהִיא הַמַּרְפֵּא.
And, it is possible that a man should commit either one grievous iniquity or a multitude of sins so that the Judge of Truth will decree against him that, whereas this sinner committed those sins of his own free will and consciously, repentance should be withheld from him altogether, and grant him no leave to repent, so that he might die and perish in the iniquity he committed. Even this is what the Holy One, blessed is He! said through Isaiah: "Make the heart of this people fat, and make their eyes heavy, and shut their eyes; lest they, seeing with their eyes, and hearing with their ears, and understanding with their heart will return and be healed" (Is. 6.10). It is, moreover, said: "But they mocked the messengers of God, and despised His words and scoffed at His prophets, until the wrath of the Lord arose against His people till there was no remedy". (II Chro. 36.16)—as if saying: "They sinned of their own free will and they have multiplied iniquities, until their guilt carried the punishment to withhold repentance from them, which is the remedy". (text and translation from Sefaria)
The Rambam goes on to give the hardening of Pharaoh's and Sihon's hearts as examples of God forcing a person to give up his free will to repent in order to be punished for previous sins.
