A few people using this plugin have reported that it doesn’t work with BuddyPress. After some investigation it appears to be because BuddyPress has it’s own registration template and doesn’t use some of the standard hooks WPMU uses during the initial signup process.
Not a big deal however in order to make this plugin work with the default BuddyPress theme you will have to make 1 manual edit to a theme file. I couldn’t find any better way to get the necessary check into their workflow but it’s a pretty simple edit.
Jump to the main plugin page WPMU Block Spam By Math, use the download link to get the updated plugin (version 1.1) and scroll down to the instructions and follow the note for BuddyPress users.
Post any issues in the main plugin post.