The story should prevent a character specialising too much in this way. Unless the hero builds relationships and other socially required abilities, they should be threatened with burning at the stake (or whatever the cultural equivalent) as their power starts to grow without restraints (and not once it is too late to stop).
If the character does get too powerful in a narrow skill then throw him into situations where their skills are inadequate and they have to burn HPs.
This obviously requires some narrator forethought but the other players should play their part too. In our most recent game, almost every character spent hero points in a competition with each other!
regards,
Charles
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