You can control this in Site admin > Messaging > Notification settings, under the Course completion option. Set the Email option to Disallowed to disable it site wide for email, or uncheck the box on the far right Enabled column to disable it completely for all types of notification. More on these settings: https://docs.moodle.org/310/en/Messaging_settings#Default_notification_preferences
General help
How can I stop system email 'coursecompletedmessage'
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