The only thing i could suggest is to stop players from getting more of the 10 and 15 enchant items. But i guess that would make a tonne of work for you, having to give players their enchant items each time they wanna enchant gear for those levels of PvP.
i dont think there is any way of "restricting" the level of enchanting how you want
Maybe set the game to automatically give players enough enchant item to enchant there gear at those levels, when they reach the level, or when they make there character.. Hopefully someone knows more info about this and knows if it is possible to do what you want