There doesn't appear to be an interface icon or means to cancel a strategic level spell (such as Evade) on your units. I can see the spell listed in each character's Enhancements area in their Details page, but no where can you right click or toggle to cancel the active spell (tried even looking at the caster for somewhere to select this). I received a message indicating that I would be running out of mana soon if I didnt cancel a spell or two, but no where is it explained how to do this.