Simply run the following SQL query on your WordPress database, using the command line client or PhpMyAdmin. This will close pingbacks/trackbacks on all existing posts.
Don’t forget to backup your database before using this query.
UPDATE wp_posts SET ping_status = 'closed';
If you like to know more about WordPress SQL queries, you should have a look to this article.
Thanks to Jeff Starr for this nice piece of code!
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WordPress tip: Close trackbacks on all posts at once