Advanced > Gameplay Constants, you will find some "HeroXPGained" rows. Change the values of "Creep Reduction Table", "Summon Unit Factor", "Hero, Constant Factor" and turn off "Global Distribution" and "Building Kills Grant XP". I don't know if all of these work, but I hope it helps. You may also want to change "Hero Maximum Level", so that your heroes can level beyond level 10.