Probably the best way to do this would be to create a trigger that regulates how much exp. a hero gets from killing a monster. Now I won't paste an actually trigger because that takes more time than I have tonight to post this but here's a little description. Assuming you have done triggers before, this should be a piece of cake.
1) First off go into "Gameplay Constants" and set Creep experience tables to 0. Set "Global Experience Distribtion" to zero or null as well.
2) Now go into the trigger editor and create a trigger like so:
Event:
Unit - Unit is killed
Condition:
Boolean Comparison - Killing Unit is Hero Equal to True
Action
If/Then/Multiple Conditions -
Condition
Dying Unit is (Type of Enemy #1) Action
Give Killing Unit (Any amount of exp you want) experience.
Else
Condition
Dying Unit is (Type of Enemy #2) Action
Give Killing Unit (Any amount of exp you want) experience.
I'm on a page about incorrect corrections, and spent the better part of like two hours trying to get someone to understand that -5^2 = -25, not 25, and then that post had comments get reposted because that group is self sustaining, and that person was in turn trying to explain what I just explained to them. And I'm taking that as a victory