Yeah, I thought of the normal event system, since an event makes other events unable to go to the same place, but Blizz-Abs don't work that way.
Anyway, I just checked and in the Blizz-Abs configuration program, on the "Enemy Behavior" tab, there is an option called "No enemy Terrain Tags". I think THAT is what you are after. It makes enemies unable to spawn on specified terrain tags, and also unable to walk on them.