I went from 100′s of blog spams a week to under 10 in the last week with a simple .htaccess addition:

# begin spammer
SetEnvIfNoCase User-Agent "^TrackBack/*" spammer=yes
Order allow,deny
allow from all
deny from env=spammer
# end spammer

This catches an evil little script with a UserAgent of “TrackBack/1.02″ that has been hitting from many IP addresses – I would guess that this UserAgent has been uploaded to vulnerable workstations or servers and acts as a bot-net..