It is possible to write custom code to whitelist / blacklist operating systems independently from the global blacklist.
This is useful if you offer Windows licenses or have other custom operating systems which are chargeable to the client.
Navigate into the directory
/modules/servers/dedicated/ and rename the file customconfig.example.php to customconfig.php. There are 2 functions inside which will help you get started. You need to have basic PHP programming knowledge to use this file.