I have recently found lot of trackback spam in one of my WordPress installation. Here is an article which will help you to remove trackback spam from SERP (Search engine Result Page).
include this line in robots.txt
My permalink structure is like this
domain name/post id/post title
Change your robots.txt according to your permalink structure.
To find the trackback urls indexed and cached by google use this :
site:[domain name] inurl:trackback
to remove the urls which are already listed in google login to your google webmaster account. From Site Configuration click crawler access. Click Remove URL. Click Add New Removal request. Paste the urls in the SERP one by one and click Submit removal Request when done. Google will remove the trackback urls in 24 hours.