[Kristoffer O]

There is one problem with my idea.

You can summon orcs in battles (Raise skeletons).

Changing the theme to shadow gate might be a solution, but there are fewer spirits than corporeal undead I believe).

Banning the use of the spell another idea (that is hard to implement as the AI would like it). Patch 2.07 will enable spell modding and allows you to disable spells.