I suggest extending the option:
"Comment author must have a previously approved comment"
with a captcha.
When a visitor comments for the first time, he will have to resolve a capthca. If he fails, his comment will be deleted (not moderated).
Just by using this option (and no Akismet or other similar plugins) blog author will have very little or no spam at all waiting in moderation queue. Just think about it...
Please vote if you like it. Thank you.