WordPress allows you to mass delete these comments in the wp-admin – but only in batches of 20 at a time. So this could be a real problem when you have thousands of pending comments.
Here is a simple way to delete all pending comments:
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1 Log In to Your Hosting Backend
I use CPanel so the screenshots are related.
When you have successfully logged in, you will see a screen like this:
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2 Access Your Database Using phpMyAdmin
Look for the phpMyAdmin icon (under databases) and click on it. You should see a page like that when you are in the phpMyAdmin page.
Select your WordPress database by clicking on it on the left. You will see a list of tables within that database.
Next, locate the tabs on the right side, and find “SQL”. Click on that.
You will see this screen.
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3 Run the Following SQL command on your database
The command is this:
DELETE FROM wp_comments WHERE comment_approved = '0'
You should see a status message saying something like “958 rows affected”.
Log back in to your WordPress admin panel, and you will find all the pending comments gone 
