Hi,
this is one of the strangest requirements I've ever heard. Why would you prevent a language to a group of users?
It's not possible. A user can select a language on logon screen. At that moment a user is not logged in so you don't know user's department. For SAP GUI connection there is a well known user exit that is executed after user logs into system. So you could check the language there and raise an error message if a user selected "invalid" language.
Cheers