Simple Trackback Validation Plugin for WordPress 2.0.x and 2.1 performs a simple but very effective test on all incoming trackbacks in order to stop trackback spam.
anti-spam plugin for the WordPress blogging platform. It is meant to stop all forms of automated Blog spam effortlessly, while remaining as unobtrusive as possible to regular commenters.
T. Yoshinaka, S. Ishii, T. Fukuhara, H. Masuda, and H. Nakagawa. Proceedings of the 2008/2009 international conference on Social software: recent trends and developments in social software, page 88--99. Berlin, Heidelberg, Springer-Verlag, (2010)