In reply to Pippin.

Yes, the SQL above does NOT do take care of deleting associated metadata, thanks for pointing it out. To make it shorter, In my initial answer I just copied one of the needed SQL sentences described in this old post of mine: http://neliosoftware.com/wordpress-database-snippet-delete-all-your-spam-and-trashed-comments-at-once/ . You´ll find there the SQL sentence for deleting the metadata as well.