You can mod the specific "Curse" and "Horror Mark" spells to cost 30 gems, and be research level 9 in all paths, if you really want to remove them from the game. You can copystat a militia onto all the units that have a horror mark attack. Or you can copystat a militia onto all the horror units. I'm not familiar with item modding but there may be something similar you can do there.
So while it's not possible to mod in a cure for these afflictions, it is possible to prevent them from occurring in the first place, if you really want to.
Personally, I think both curses and horror marking add to the game. "Curse" will make you think of a new use for a thug you've been using to slug it out on the front lines. "Horror Mark" makes you think of a strategy to keep a unit effective day to day, but also able to defend itself against a horror attack. Or for uber items which horror mark, you'll be more likely to use them with a disposable unit instead of putting them on a unit which you don't want to die.
But all units can die in this game anyway, insta-death (or effective insta-death) can occur to any unit. It's not Sim City
.