This letter is available here . Well then, let's take a look at possible methods of blocking. Normally, users use the DNS of their respective ISP because of speed, but are in no way obliged to.
Any user can use any DNS in the world.
Furtheron, any user can bypass the DNS if he knows the IP-address already. Time needed to block is about 5-15 minutes per domain. In this case not only the lookup but the actual site really gets blocked. Any attempt to transfer data directly to or from the blocked site will fail. A heavily used method to avoid traffic in the internet is called proxy.