I guess you have not seen the curse 'ability' yet have you?
http://dom3.servegame.com/wiki/Curse
If you want to remove horrormarking you need to remove all the items and spells that horror mark. (There are a few items, mostly artifacts). And a few spells that either summon horrors, or give horror marks. (the last are all astral).
It is not possible to cleanly remove the horror mark abilities from some of these spells/items, you must remove them entirely. (horror marking items are hard to mod for example). Blood will be crippled. (As will certain abysia nations, as you remove the entire blood+astral and research bonus part of the nation).
Personally, I think you are crippling the game by doing this. Just because of personal issues with permanence and loss
.
Also, horror marking is one of the many ways you can fight SC's. It is an equalizer. Horror marking also matters little on normal units. Only commanders get random horror mark attacks.