I’ve intended to disable all comment or trackback functionality to avoid having spam and dealing with it in form of anti-spam plugins like Akismet. While I already had comments disabled, Trackbacks were still active.
Even when you disable Trackbacks in the WordPress settings via „Settings / Discussion“ under „Allow link notifications from other Weblogs (Pingbacks and Trackbacks)“, your existing pages needs manual update to take effect.
Connect to your MySQL database of your blog (e.g. with PHPMyAdmin) and execute these two queries:
UPDATE wp_posts set ping_status='closed' WHERE post_status='publish' AND post_type='post';
UPDATE wp_posts set ping_status='closed' WHERE post_status='publish' AND post_type='page';
I’ve found this information on Andreas blog, thank you!