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Hidden potentials are unlocked when a character has earned enough (what people call) Ellet Points (you get those by deploying a character in battle and having him perform actions) and then visiting Castlefront Street where Ellet says something about interviewing Squad 7 raising star.
So, in order to keep hidden potentials locked, you need not to do those. Meaning, either never get enough "Ellet Points" or never visit Castlefront Street (still you may want to visit it to buy everything there is to sell and unlock the other hidden potentials you want). Another solution would be to edit your save to lock Humanitarian again (or replace it with something else).
What I did, personally, is go through the game and new game + without ever using a character with a negative hidden potential (except Oscar and Karl as those are required to unlock Emile and Lynn) and while unlocking all positive hidden potentials. After that, I went to Castlefront Street for the last time to buy all Ellet has to sell (and never visited her ever gain). And thus I ended up with a NG+ save (meaning all battles accessible) with all positive potentials unlocked and no negative hidden potential unlocked.