You wrote this in the passive voice; it doesn't say who is doing the blocking.

Pornhub itself is doing the blocking; it uses geolocation and denies services to IP addresses from jurisdictions with age verification laws. The laws are usually not structured so as to require a third party such as an ISP to block noncompliant sites; instead, the governments of the states with those laws can sue the porn sites and their service providers (Verisign in the case of .com domains).