If you are willing to really upend the game balance, you could just rule that your world has no healing magic.

People who are immune to magic (or psychic abilities) for whatever reason are a fairly common trope outside of D&D. They make excellent witch/demon hunters, as well as having a major leg up in hunting any creature that uses magic.

There was a game a while back where everybody had what a stat for how in-tune they are with magic. Higher-score characters can use magic (and higher-score = more powerful magic), but also made you more vulnerable to hostile magic. At the other end of the spectrum you had people who were so magically insensitive that they couldn't even PERCIEVE magical effects, but they also couldn't be effected by them.

If the plot device you are going for is that a specific person can't be healed by magic, maybe that person is a magical null, or comes from a family of them.