I have written (due to an increase in attempts to register on my sites) a standalone plugin similar to stop spammer registrations. This one hooks the login and signup forms and checks the SFS database for IP only. It denies access to the form for anyone who is a hit.
I am getting about a dozen signup events blocked each day.
I use the plugin to help me hand craft my htaccess file. What I do is check the plugin for the IP address of those trying to sign up and I manually add a "deny from" to my htaccess. I am slowly building up a list of the bots hitting my site. I record both failed and passed attempts in the plugin so that I can check the passed IPs to see it they later appear on the SFS db.
You can download the testing version of the plugin at:
I will release this as a standalone plugin in a few weeks and then I'll roll the functionality into the stop-spammer-registrations plugin when I find time to work on the next version.