Hi,

Use this hook code: 

add_action("wpdiscuz_before_wp_new_comment", function ($commentdata) {
    if (!empty($commentdata["user_id"]) && !empty($commentdata["comment_parent"])) {
        $parent_comment = get_comment($commentdata["comment_parent"]);

        if ($parent_comment instanceof WP_Comment) {

            $parent_comment_user_id = (int)$parent_comment->user_id;
            $comment_user_id        = (int)$commentdata["user_id"];

            if ($parent_comment_user_id === $comment_user_id) {
                wp_die(esc_html__("You can not reply to own comment!", "wpdiscuz"));
            }

        }

    }
});

Helpful article: https://www.wpbeginner.com/plugins/how-to-easily-add-custom-code-in-wordpress-without-breaking-your-site/