I resolved by simply removing the content of "Author label" field in Settings > Comment Form.
This can't avoid direct access to the subscriptions manager page by tech savvy users, however. To avoid this, I edited the plugin core file "author.php" and added this at the top:
wp_redirect( home_url() );
So if anyone tries direct access, he's redirected to the home page.
These solutions blocks the admin also, but he can always manage subscriptions in the admin area.
I know that editing plugins core files is really wrong, but I'm not able to find another way. Anyone has a better solution?