Trackback spammers have recently been flooding blogs with spam, often with hundreds per day. Although MT's Spam Lookup plugin junks most of the spam, the flood of trackbacks can put a strain on server resources. To take a look at the amount of trackback spam you may be getting, click on "Trackbacks" from your main blog menu, then select "Junk Trackbacks".
Listed here are some defensive measures you can take.
Moderate all trackbacks
MT 3.2 allows you to approve all trackbacks before they post to your site. To do this, go into your blog's "Settings" and select "Feedback". Scroll down to the TrackBack section and check the checkbox next to "Moderation". Save changes.
MT 3.2 ships with a powerful anti-spam plugin called Spam Lookup. If you are getting hit by a flood of trackback spam, look for the common unwanted words or specific strings to block. If a spammer is leaving URLs like http://yucky.nasty.com and http://icky.nasty.com, all you have to do is block "nasty.com". Do this by adding the domain name in the Plugins > SpamLookup > Keyword Filter Settings > Keywords to Junk "Keywords to Junk" in .
Spam Lookup makes use of PERL Regular Expressions. So by adding a few characters to your keywords you can have more flexibility in what you block. (Personally, I have no idea how to use Regular Expressions. If there is someone out there in the community who is willing to walk me through the basics and what one would most often use with Spam Lookup, I'll be happy to write it up.)
Neil Turner has written the tutorial Making the Most of SpamLookup which explains more about this plugin.
Akismet is a distributed spam filtering system that can be used to fight comment and trackback spam.
MTAutoBan - prevents the same sources from filling your database with junk.
TrackBack patch for MT3.2 - changes the trackback API to use identifying strings instead of numerics as the trackback key.
SpamLookup Extension - This modification to SpamLookup provides the ability to apply word filters to specific fields in the comments and trackbacks, rather than the conglomeration of all fields.