Playing whack-a-mole with extremist communities is only going to drive them together under a common threat and make them more desperate. The dedicated ones install a tor client and go further underground. The not-so-dedicated ones leave. But the not-so-dedicated parts of the community are the ones helping contribute to the deradicalization of the others.
It should be obvious to state that if you get rid of 8chan, those people aren't suddenly deradicalized and they're still in the country holding the same beliefs. Those people who are today at risk of continuing in this shooter's footsteps have already read the manifesto. People like the shooter often do these things because they want to be heard and they want to contribute to the course of history. Taking away what little voice they have in their own spaces makes them feel less heard. Destroying the little community that they have removes their stake in the world. Suppressing their only place to express their grievances causes them to lose hope. This is the cocktail for more violence, not less.
The *chans are the furthest from centers of indoctrination because the moderation is the weakest of all social media platforms. All ideas are present and little to nothing is suppressed. Dissent is commonplace and general consensus has no power. Sure, it means ugly ideas get spread, but it means that people actually have to refine their moral argumentation. No longer can you assume that the other person has the common ground of "racism is wrong", you have to actually dive into why racism is wrong. You can act outraged all you like, but it won't convince people to change their mind. Changing minds is going to be the only effective course of action to avoid tragedies like these.