Making a DNS server not respond to queries for a specific name is trivial for any DNS provider to implemen
It might not be that easy, you’re thinking about one single server running some kind of DNS server you’re familiar with. When we’re talking about Quad9, Cloudflare etc. were talking about hundreds of servers across the planet, highly distributed solutions that rely on multicast and other non-trivial techniques. If you’ve to change a system like that to add the ability to block something, trust me, it won’t take a few hours.