I would recommend looking into an http blacklist such as
[url]http://www.projecthoneypot.org/httpbl_api.php[/url]
I utilize this project for my personal forums to verify the authenticity of an ip and whether it has been associated with spam/dictionary attacks/comment spammers. The project has been very beneficial and may prove to be an extraordinary tool to combat the issue.
I'm not in any way affiliated with the site, I'm just grateful it's there.
