i think he wants to be able to train the unit and then not be able to revive it. then you would do this:
Events Unit-A unit finishes training a unt Conditions-trained unit equal to [whatever the hero is] Actions-set training availability of [your hero] to off or somthing like that. i think this will work i didnt test it though
Easiest thing to do there is use the "campaign" heroes, you can build unlimited of them if you allow them to be trainable at an alter or barracks. (you can do that with the normal heroes, too, I just don't remeber how to do it off the top of my head.) And simply remove the "revives dead heroes" ability off the Alters/Taverns. (might be a true/fals box you have to check) -VGsatomi