Recently I happened to encounter a WordPress blog which has more than 10000 spam comments and most of them are approved automatically,so my task is to delete all comments at once(including genuine comments),but if you do this via WordPress GUI it would take hours to do the job so here is a easiest method to delete all comments at once using Phpmyadmin.
delete from wp_comments WHERE comment_approved ="0";
To delete all approved comments
delete from wp_comments WHERE comment_approved ="1";
Instead you can use this command to delete all the comments
delete from wp_comments;
You can execute these commands via Terminal also.but please have a Database BACKUP before you do this.I recommend to use free spam protection service called Akismet to avoid spam’s in future.