You can't, at least not for closure flags. You could still flag it for another reason (spam, or for moderator attention) but those are usually not appropriate.
But does it really matter? The effect of the closure flag is to send it into the Close Votes review queue, and it will take five (on some sites, like Stack Overflow three) other users to actually close the question. Your close reason does not have a direct influence about what's shown in the banner when the question get closed (though the reviewers will be able to see your choice).
If you're really worried about it, leave a comment explaining the actual reason you want to close the question. The author may even improve it sufficiently for that comment alone.