The solution for disabling the possibility to add identities by adding this to the roundcube-config
The roundcube path may vary depending your OS and where it's been installed by the control panel you use. Some paths you can try are:
(/usr/local/cpanel/base/3rdparty/rouncubemail/config/config.inc.php):
(/opt/www/rouncubemail/config/config.inc.php):
$config['identities_level'] = 1
The different levels of roundcube are:
Set identities access level: