This plugin schedules a daily event using wp_cron that deletes all comments marked "spam" in the database, then optimizes the comments table.
Brief reason why I made such a low-tech plugin:
Akismet catches all the spam, but when I was looking at the size of my backups one day, I noticed that there were large numbers of comment spam across several sites that were undeleted and they made the databases fairly large. So I setup a cron job on my server that deletes the spam.
Since I had never written a plugin, I thought this might be helpful for those who do not have a dedicated server, shell access to their site, or the knowledge of (or interest in ) cron to set this up.
Spam is not deleted until you start the schedule after the plugin is activated. I created buttons to start and stop the schedule if you need that layer of control. There is also a button for deleting all spam manually.