I had trouble finding this at first too. I think they should add a shortcut in the Spellbook that takes you there, or a little button in the minimap next to spellbook.
Anyways you can dispel your enchantments by going to the screen where you can see the Ledger, Design Units, Kingdom INfo. One of those buttons shows all your current spells and their upkeep. Sorry but I dont remember the exact name of that screen but that should help you find it. Hope this helps.
