You may try this
Simple Trackback Validation Plugin
It eliminates spam trackbacks by retrieving the [COLOR=blue ! important][COLOR=blue ! important]web [COLOR=blue ! important]page[/color][/color][/color] located at the URL used in the trackback and checking if the page contains a link to your blog: if not, the trackback is placed into comment moderation queue.

Download and Install Instructions can be found here.