You can uncheck Settings >> Discussion >> “Allow link notifications from other blogs (pingbacks and trackbacks) to disable disable for future posts. However, to disable for exiting posts you need to run a SQL query on your database to turn the ping_status to OFF:
UPDATE wp_posts SET ping_status="closed" WHERE post_status="publish" AND post_type="post";
and
UPDATE wp_posts SET ping_status="closed" WHERE post_status="publish" AND post_type="page";