Do people really have such little self-control that they need to block a particular internet site via a tool like this? Just don't type news.ycombinator.com or click your bookmark for it. Simple as that.
Breaking that habit definitely helped my cause.
There have been internet communities that I've been a part of that I have been emotionally bonded to. You get to know some people well and suddenly it's more a social experience than intellectual. That's a completely different story, and you'd need more than self-control to stop going there if it was becoming a time-suck.
Most people that visit sites for new content get a tiny burst of pleasure. It's the same reason that people gamble, and the impulse is strong.