I've had great luck with Spam Be Gone and Captcha's with keeping spammers out of my old site running on SMF. The initial problem though was weeding out all of the spammers that had signed up before I implemented it. I probably manually deleted 2000+ accounts. After that was taken care of, I never found a need to look at individual posts automatically. The ones that slipped through were flagged by users or a moderator deleted it and the associated account. The Spam Be Gone plugin prevented over 130,000 spammers from creating accounts in the 4+ years that I used it. It was very effective. I did try Akismet for awhile, but got sick of paying for it. I didn't notice any reduced functionality when I got rid of it. I would try Spam Be Gone, use Google's v3 captcha ("I'm not a robot." checkbox), and then make some of your trusted active users into moderators with the ability to delete posts/topics/accounts.