After mistakenly choosing an action, or deciding to use a different action, there's no way to cancel a previously selected action.
This is somewhat related to the items brought up above in that once we change over to a gameplay style that allows for immediate execution of commands, this should no longer be an issue (since there won't even be an opportunity to cancel something to begin with).
Thanks for reposting, Davorok! I forgot about this.

"There won't even be an opportunity to cancel something to begin with". Does that mean all spells will be instant cast? Hrm.