2 Ways:

1- Fast: Go to city details screen. On top of the left, you will see circles with the enchantments. On top of the circle, there is an "X" that will remove the spell.

2- Alternative: Go to "Govern" (bottom, on the right). Then hit in "enchantments", on top in the middle, and you will see the list of them. Click on "dispel" the one you want to remove.

If you want to remove a spell on a unit, you will have to use the way 2 (I really miss way 1 for removing spells on units)