you can unhook the email action by placing this code in your functions.php:
remove_action( 'woocommerce_order_status_completed_notification', array( $email_class->emails['WC_Email_Customer_Completed_Order'], 'trigger' ) );
you can unhook the email action by placing this code in your functions.php:
remove_action( 'woocommerce_order_status_completed_notification', array( $email_class->emails['WC_Email_Customer_Completed_Order'], 'trigger' ) );