Check the ./config/main.inc.php and modify the following setting:
// automatically create a new RoundCube user when log-in the first time.
// a new user will be created once the IMAP login succeeds.
// set to false if only registered users can use this service
$rcmail_config['auto_create_user'] = TRUE;
to
// automatically create a new RoundCube user when log-in the first time.
// a new user will be created once the IMAP login succeeds.
// set to false if only registered users can use this service
$rcmail_config['auto_create_user'] = FALSE;
Now no new users can access their mail using your RC environment