The maximum difficulty a monster can be generated at is independent of player experience level. Only the level difficulty (and perhaps other player-independent stats such as branch) matter. The details of how the curve of monster difficulty compared to level difficulty should look are up to debate; NetHack4’s is a piecewise function that climbs slowly until level 10 and thereafter is (level difficulty - 5).

As a more minor version of this, only enforce this rule during level creation, and respect experience level for monsters generated after creation.