With only the FTD you will not be able to block devices based on operating system information. You would need to implement an identity access management system, like ISE, to be able to do this.
You can, however, integrate the FTD with Active Directory or LDAP and deny access based on user info. So if you know a specific user has a windows 7 machine, you could deny that person reaching the internet. But in this case you would also be denying this person access to internet from all machines they log onto with that username.
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