What could they do other than block a target's DNS resolution?
You can only block name resolution on that country's ISP nameservers. The Russians can use Mindgeek websites -- VPN, Proxy, tor ...
You have to remember the monetization. Ad revenue -- American pay site, cams, other porn related ... buyers can be blocked on a VISA-Net level but bitcoin wallets would work-- albeit-- most revenue from the USA could be lost.
Depends on what priority "keeping our children safe from Porn" takes
