Seems that some spammers have found a way around the captcha tool. We get pounded by a few spammers that result in email notifications. Marking a comment as spam does nothing to prevent repeated abuse.
Marking a given comment (posting email address) as spam should prevent further commenting.
Yes, there seems to be no point in marking a comment as spam. Ideally, when you mark a comment as spam, all future comments with that email address would automatically be marked as spam. It would also be nice to be able to maintain a custom list of words that automatically mark any comment containing those words as spam.