I have another post in the forum, but if you are interested in using a great software package that limits spam and server attacks before they access the site, then you might be interested in Bad Behavior. I really like it because it really decreases the load on the server as well.
While they do not have a plugin for bbPress and creating one isn't in the cards right now, you can do it by downloading the newest Bad Behavior release and placing it as a folder in the root of your bbPress installation.
After that, you just need to call it. I did this by adding the following to config.php after the /* Stop editing */ part.
require_once( BBPATH . '/Bad-Behavior/bad-behavior-generic.php');
That is the easy way without a plugin. Just thought I would share it for those that are interested.
Trent