Hi Zee,
Don't want to appear to be a busybody, but I was wondering how things are going with blocking the spam. It must be an absolute pain in the backside for you to deal with.
I've just had a little search on the net and here is an article (for others who are interested as well) that might throw up a few ideas http://www.sitepoint.com/article/stop-comment-spam
Perhaps for a time the site might need to be less user friendly? or perhaps if a bot is being used, might need to relocate the forum to a new directory. Don't really know how this might work or cost? in whatever set up is being used.
Perhaps it isn't much of a problem to people. Anyway know that if I can support your efforts in stopping it then count me in, even if it means more inconvenience.
Gordon