brockangelo

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.

Delete Spam Daily

Uses wp_cron to delete comments each day that are marked "spam" in the database.

Automattic

Mohammad Jangda

Erick Hitter

Thorsten Ott

This plugin allows you to take control over the execution of cron jobs. It’s mainly useful for sites that either don’t get enough comments to ensure a frequent execution of wp-cron or for sites where the execution of cron via regular methods can cause race conditions resulting in multiple execution of wp-cron at the same time. It can also help when you run into posts that missed their schedule.
This plugin implements a secret parameter and ensures that cron jobs are only executed when this parameter is existing.
<h3>Limitations</h3>
This plugin performs a <code>remove_action( 'sanitize_comment_cookies', 'wp_cron' );</code> call in order to disable the spawning of new cron processes via the regular WordPress method. If <code>wp_cron</code> is hooked in an other action or called directly this might cause trouble.

WP-Cron Control

This plugin allows you to take control over the execution of cron jobs.

Delete Spam Daily vs WP-Cron Control