Generally in user interface design, it is considered bad practice to present the user with choices and not also allow them to cancel their decision. This leads to feelings of being trapped by the game into making one of several bad choices, despite the “teaching a lesson” aspect of planning carefully before selecting. I am in favor of a game design that supports canceling of half-cast spells, even if it gives an “unfair” opportunity to cancel a targetable spell when a similar untargetable one would not have such an opportunity.