Disabling individual sites on a Craft 3 multisite
Seems like this is something you'd do from your .htaccess file or public index.php files for each of the non-primary sites you define.

For example if you have a public_html/index.php for your English primary site and a public_html/de/index.php for your German site, your German one would just header("Location: http://mysite.com/503"); to redirect.