IP banning doesn't really stop them, since there a virtually limitless number of IPs that can be generated for the spam sources to continue their activity from. The forum does catch most of them before they can even make posts, and staff can take further steps against them using the StopForumSpam system we have in the forum's controls. One of those functions is an automatic ban of the IP and associated IPs of a recent spam account.
The number has gone down in recent history, at the very least. If there are further developments in our prevention capability an update will be provided.