Even if you’re careful when approving new comments, sometimes you might just forget to visit the commenter url and detect a long time after that the linked site is spammy. Here is a very easy way to bulk delete all comments with a specific url, using a simple SQL query.
Just run the following query on your WordPress database to delete comments with nastyspamurl as url. Don’t forget to replace the table prefix wp_ if your database is using another prefix.
And of course, create a backup of your database before running the query!
DELETE from wp_comments WHERE comment_author_url LIKE "%nastyspamurl%" ;