The ultimate plugin and service to stop and kill splogs in WordPress Multisite and BuddyPress, from WPMU DEV.
No, this plugin is only compatible (and useful) with Multisite installs.
A current WPMU DEV membership is needed to get an API key to access our powerful splog-checking service. It is not required however to use the many additional signup protections this plugin provides.
Here are some of the signup protection features you don't need an API key for:
Anti-Splog is fully BuddyPress compatible with the exception of auto-renaming wp-signup.php. Note that it does not yet protect against spam users or their entries in status updates, forums, activity streams, etc.