As of its most recent version, b2evolution has a problem with trackback spam. Several b2e users (including Whoo and Graham) have suggested renaming the directory that contains (among other things) the trackback script. In theory, this should eliminate spam from bots but should not effect legitimate commenters and pingers.
Here's a perl script I wrote to automate Whoo and Graham's suggestion: hidehtsrv.pl. I suggest creating a cron job to run hidehtsrv.pl periodically. Note: this script requires another of my perl scripts, randpass.pl, a pseudo-random string generator.