hate to say it, but it might be a good idea to maybe put a simple verification check into the forums, beyond the jpg image? 
Unless, this happens all the time and I haven’t see them
As best I can tell, most of the spam is human-generated, so it’s really hard to get around. My guess is the spammers have sweat-shop laborers doing their spamming since most places now have verification to get around spambots.