Isn’t that more or less what happened in Australia after the Port Arthur massacre?

On the other end of the spectrum, Allende basically sealed his fate by disarming his own supporters.

Regarding your first point, I think there’s more harm mitigated than rehabilitation opportunity lost by quarantining and thus limiting the outreach potential of extremists. The saner, less edgy parts of the Internet are easy enough to access and these people will come across them in due course if they are in a receptive mindset. Whereas I am completely comfortable making their weird little dens of groupthink harder to stumble upon by default.