Removing 'Stock' hero icons from Altar

I'm working on a map where each of the two players have 8 custom heroine high elf rangers and 1 custom heroine sorceress. I have no need for the three "stock" human heroes. I've tried deleting them from the available units in the Unit editor, but that has no effect. When I build an altar, the 3 stock icons still appear - taking up the bottom three icon slots in the altar. The problem is, with them taking 3 of the 10 available slots, if my 9 custom heroines get wiped out at once (or before I can start resurrecting some of them) two are gone. Since there is no slot for their icon in the altar - there is no way for the player to resurrect them.

I suspect I'm missing something very obvious here, but for the life of me, I can't see what it is. Anyone know how I can remove the 3 hero icons that are standard stock and fare with an altar?

In the altar properties there's a trainable units field -- remove the heroes.

That's almost too simple. You'd think after spending 40+ hours on this map, tearing my hair our trying to learn about triggers and how to write them, I'd have looked at the building properties - but in fact, I haven't had a need to do anything with the buildings and the thought never crossed my mind.

So it was as obvious as I'd feared ... duh! Thanks Dataangel.