Echo chambers cause an overall increase in extremism. After reddit banned /r/fatpeoplehate, reddit as a whole became less mean.

When you give extremism room to congregate, extremists feel more empowered. (This is generally true, but relevant only to extremism at the moment).

Unless you simultaneously cut them off from the rest of the site, which in the context of reddit, twitter, and Facebook doesn't make sense, you just make things worse.