A couple of points for you guys to consider. First time poster here, But I manage some large forums that rank for lots of high value keywords.
Our line of defence on vBulletin is as follows:Registration - Passed to StopForumSpam (this allows us to gauge the threat of the email address used and the IP address used). New users are not allowed to post links - configurable post count. Posts are passed to Akismet to gauge their spam levels.
Even with this, we still get hit every now and then with manual spam from SEO agencies in various parts of the world - some parts of the world are worse than others. For those we use broad CIDR bans at the firewall levels.