there's a list of all the troop and city enchantment. either click on the "mana" section of the resource indicator bar at the top, or go to the govern screen and click the "enchantments" button at the top of that dialog. you can then dispel the enchantments of individual units.
there's already a suggestion in the community UI suggestions thread to change that mechanic (so you can cancel the buffs directly in the unit screen). feel free to bump that thread and write a comment if you'd like that changed