To slow down the leveling, I don't know of a quick fix.
It doesn't matter that much, though, because in Enderal the enemies do not level with, unlike Skyrim, there are easier and harder areas.
You also determine the level of difficulty as well as your used level(learning) points. (also if you want to retain your points)
The only thing they can't delay is their level-up bonus, be it magic, stamina or life points.