If your blog is very old and if it receives good number of visitors, then, you may receive thousands of comments per month. There are a lot of anti comments spam plugins available out there like Wp-SpamFree and Akismet. All these are pretty powerful automated spam blocking systems. But, what if the readers are submitting comments manually. There’s an option to CLOSE the comments in WordPress, but that will stop the legit comments too for your post.
Actually, I turned one of my old blogs in to an article directory as I was not having time to do create posts in that blog. Now, when I logged in after a few months I saw about 1500 pending comments in a week. WordPress allows you to delete 20 at a time. At that rate if I have to delete the comments, it will take probably an hour to do that. So, the solution for this problem is a WordPress plugin: Delete Pending Comments.
Just install and activate the plugin. You need to make sure, before doing this you have manually approved the relevant comments. Since, after this action, all the comments will be deleted. Once you have approved then, click on option “Delete Pending Comments” just under your comments section. Type the given text in the box and click on “Delete Pending Comments”.