I could see different ways to lower thralls / influence from Sauron:
- the wizard would some sort of spell to brainwash the people into worshipping/adoring or at least protect them from the influence of Sauron aka SK. This spell could be upgraded to a grand spell of brainwashing reducing thrall globally in all controlled cities and even sway minor factions away from SK towards you.
- the tyrant would have some building like a torture chamber / secret police (expensive unit) or the like to force the people into obeying him
- the guardian would have some kind of terrain protecting spells to keep the SK influence from the land. Maybe the spell protects a few number of tiles. Depending on the proportion of controlled land protected vs. the total controlled land, it could reduce the thrall by that proportion. Ex. a fully protected controlled land would remove thralls entirely, while even a small unprotected tile would incur one thrall in each city or something like that
- the tinkerer would be able to create an artifact from crafting recipes/materials which radiates obedience/admiration
- the warlord would be able to plant one or more banners in the cities or in the land to rally the people into fighting for him. however there might always be pacifist, which should be put to the sword....
- priest would have some kind of worshipping shrine/temple, maybe even a sacrificial alter aka Aztecs... depending on the intagibles (evil). Of course the priest would eventually be able to cast the grand spell of brainwashing like the wizard.