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).
I completely forgot about this. I really hope they are not planning on making every spell instant cast. Some instant cast spells are ok, but they definitely should not all be instant...