In a number of issue areas regarding Internet content, law enforcement is already actively engaged in pursuing the people who make the content available. I understand that in those areas the content is moved around rapidly. For people who are sophisticated and have the intent to circumvent a blocking regime, there are ways to do it.
For an ISP to do it, again, it's very difficult. There's a technique called “DNS poisoning” whereby you simply take the address out of the domain name system tables. The problem is that it's as easy as changing the domain name to make it reappear. And, again, the people who do this kind of thing are very well versed in how to do that.
Another way to do it is by blocking IP addresses, but that often has unintended consequences because websites often are not the only resident at a particular IP address. Blocking an address will often have collateral damage results to other content on the Internet. That results in a situation of having content overblocked, which, again, is not a result that is generally considered desirable.