This is solvable.

First: don’t tie transformations to the moon cycle. Every night they roll a Wisdom save, and the DC increases each time they succeed. If they fail, they’re a bear/wolf/whatever and go around being a jerk that night. They still have a sense of when their time is coming up, but not exactly when, and they also don’t pester you asking about the moon every night. Transforming in town is Bad News, no matter how nice a bear you are, and this means you can never really be sure you won’t.

Second: enemies aren’t stupid. If their weapons aren’t working on this guy, get him out of the fight some other way while we go punch his friends. He’ll get real annoyed that enemies just keep using crowd control on him, and his friends will get annoyed that they’re getting creamed because damage isn’t being spread out as far.

These two things together means that there are real consequences for not seeking a cure for both the characters and the players driving them.