Heres how you do what Tobbzn said:
At the top of WE go to Advanced tab -> Gameplay Constants...
tick "Use custom gameplay constants" then go down to Hero (or just type H)
There should be one saying "Hero Attributs - Attack bonus per primary att point"
At the moment it should equal 1, this makes it each time the hero gains 1 primary attribute point
(strength, agility, intelligence) it gains 1 damage, simply change this field to 0.
This way the hero still gains other attribute bonuses yet doesnt add damage, you can edit the other fields as well if you need it.