I'm still not sure you understand; none of your players are "over powered", least of all the Monk. As stated twice already, the paragon path he's in is a garbage one for a Monk. One decent ability usable every two combats do not a munchkin make.
Tell your Monk player you're sorry for overreacting, and just let it be.
Stop comparing the Monk to your other players (and your other players to each other!); if they feel they're underpowered, then let them pick a different paragon path, but rest assured, the monk is not going to be breaking anything in his current path.