[Edi]

The sites that cause horrormarks and curses can be overwritten, but then you lose the other properties they may have. Site modding does not allow assigning horrormark or curse giving properties or many other things, so overwriting is the only way.

Otherwise, you're in for a lot of modding to remove or modify all the spells that can give horrormarks or curses. Those things are not moddable out of items either. Horrormarks are reduced somewhat every time a unit dies, but that's not much comfort most of the time.