bbPress supports multiple forums by default, and does not include options to support a only a single forum.
Solution (redirect hack):
Replace the contents of your template’s front-page.php with this code to permanently redirect the forums site to your preferred single forum. “http://www.example.com/forums/forum-slug” must be the direct URL to the single forum you want to show:
< ?php header("HTTP/1.1 301 Moved Permanently"); header("Location: http://www.example.com/forums/forum-slug"); exit; ?>
This will permanently redirect users from the frontpage to the specific forum.