Wow flooded this morning!
When I ran a forum, I found the spam bots were targeting known script filenames. Simply going through the php scripts and changing the filenames was enough to break their scripts 100%. I had to search each php script for invocations of the modified account and posting script names. The spam scripts are written to target thousands of sites. I bet if each site were modified slightly, forum spam would cease to exist.
The spam script most likely searched for the string "Powered by SMF 1.1.8 | SMF © 2006-2008, Simple Machines LLC" at the bottom of the page and added this site to their list sent out to botnets.