While Akismet does a good job at flagging comments as spam, it by default only purges spam (from the comments and comments_meta tables) after 15 days.
So it’s a good thing Akismet now has a filter to change the amount of days after which spam is removed. Below code (in a small plugin or in a child theme’s functions.php) should do the trick.
/** tell akismet to purge spam sooner */
add_filter('akismet_delete_commentmeta_interval','change_akismet_interval');
add_filter('akismet_delete_comment_interval','change_akismet_interval');
function change_akismet_interval($in) {
return 5;
}
Happy purging!