My intuition would be to click on the champion you have cast the spell on, and see if there is some sort of option on him/her that would let you remove the spell.  For example, if you can see an icon of the spell somewhere, try clicking on it, or seeing if there is a small X near it or the like.  Trying to remember from the original WOM, but it's been a few months since I booted it up. 

Hope that does it, sorry I don't have the beta myself to poke around and answer for you (but if I did, I wouldn't be looking at the forums anyway )

Which works but it's not exactly obvious to the user

Here is the unit. He can cast Shockwave, a spell he cannot normally cast because I cast Hymn of Torax on him.

Now here is the spell, which I cannot cast again because I already cast it on him.

And here is my enchantment list