K-means clustering.
This will scale, is compatible with machine learning, and yields an effect that is good for the users of a service -- but is not useful to owners of a service that maximizes profit and control over user happiness.
If trolls, jackasses, and idiots (by each users independent definition) get automatically segregated into groups where they see only each-others posts, and out-groups just never see their posts: problem solved.
Now, you might say "I don't want a service where neo-Nazis congregate!". I say: why, again, don't you want your police service to be able to infiltrate these groups and observe these people, and arrest them as they see fit?
Hmmm. I wonder.
Once it becomes clear to "evildoers" that the site is not "friendly" to their evil -- toleration doesn't equal agreement -- and that they are not hidden from justice, they will go elsewhere (at least the smart ones), and you can round up the dumb ones at your leisure.