I don't think it's possible to say that social pressure, or in other words, education, is definitely going to fail. Harassers are people. We need to teach boys that they are not, in fact, entitled to women's bodies, and hopefully then not just online harassment but also street harassment, sexual harassment, and sexual assault will decrease.
But what's wrong with blocking 4chan, or gamergaters.com or whatever else? These are sites that are demonstratably, reliably causing people gross personal distress and sometimes egging people on into massive acts of violence. Notably, the UC Santa Barbara shooter was a 4chan member, and almost certainly was radicalized there.
It's easy to say that "free speech" or "censorship-free internet" are good things; it's much harder to look at the actual statistics and realize that every post on 4chan is a little piece of an inevitable hate crime. The average is the total over the number; regardless of the small average contribution of each 4chan post to harassment and violence, the end result is GamerGate and mass murder.
Ironically, when it comes to Muslims doing the same thing as white men on 4chan, the media and hacker news is united in wanting to shut down "radical Islamist forums."
At the end of the day, you have to decide what's more important: an internet where no website is blocked, or the safety of women. If you decide the former, you are every bit as responsible for the deaths of women as the people who physically pull the trigger, because you said nothing and did nothing when you could have stopped the root cause.
As another point, there is massive value to the survivors in making it absolutely clear that what happened to them is not okay; that they did not deserve it; that they should be outraged and that the perpetrators, when caught, will be punished and ostracized. The only way to do this is by actually condemning, punishing, and ostracizing perpetrators. The benefits of solidarity are very real; women who experience victim-blaming after sexual assault have vastly higher rates of PTSD.
So societal pressure in favor of feminism and away from male entitlement seems like a fine way to address this problem, and if it leads to the death of websites that organize harassment campaigns so much the better.