4chan and Kiwifarms Sue Ofcom over Attempt to Enforce Online Safety Act in US

Tl;dr can’t 4chan and KiwiFarms block us users? Or the other way around?

I think you mean UK users... and yes, they could, but they're not required to by law, and why should they do so just because some foreign bureaucrat demands they do it?

By contrast, Ofcom could've simply ordered ISPs to null-route 4chan and KiwiFarms and that would've been that. Instead, they decided to do the most ineffectual and imbecilic thing possible... arguably because they're an extortion racket (as implied by the fact that they're mostly funded from fines and fees they obtain from those they prosecute).