In the past I got a lot of trackback spams. Thats normal, no worries about that. But although the spam are identified by s9y I have to delete it by myself. So when I get tons of spam daily it is not that fun on the delete session.
My suggestion: Why not "moving" all identified spam into its own "folder" (by changing a flag in the database) and delete all entries in that "folder" automatically if they are older than X days? This delete action could run on a request for serendipity_admin.php?serendipity[adminModule]=comments because it doesnt have to run more often. Also it is not that expensive in that way.
The idea behind is only to get active if a false positive comes in and not on every incoming spam.