Most FOSS is written in English, so just ban the Bad Guys (tm) from speaking English, simplez!
But more seriously, you have two approaches to restricting FOSS: one is to deny its use via a license, but who will enforce that in those countries? The second is to deny access by having a "Great firewall of the West" but we know how hard that is to make work against anyone smart enough to code, and the simple option of blocking whole countries at the BGP would allow authoritarian regimes to further control their people's minds by blocking a view of the rest of the world. Neither would do much good.
Really the main sanction that can usefully be employed against them is to block the supply of hardware needed to make any of that work, and to deny direct support (as most companies did by withdrawing from Russia after Putin's invasion of Ukraine).
Biting the hand that feeds IT
