The same thing we use when enforcing the rules of any infrastructure: publicly accountable and transparent due process. These problems will continue until big tech implements a proper rule of law[1]. They will continue[2] to be responsible for these difficult decisions as long as their platforms idea of justice is the rule of man[2].
[1] https://en.wikipedia.org/wiki/Rule_of_law
[2] https://www.gdcvault.com/play/1024060/Still-Logged-In-What-A...
That's not what I'm talking about. The platforms need to create a new form of "legal process" where these issues are defined and adjudicated in an open process. Google et al are de facto a new layer of government, and they need to act like it. Currently, they are trying to act like a "(mostly) benevolent dictatorship" and finding out that the hard way that you can't actually please everyone. The proper way to handle calls for censorship/banning/etc is to tell them to take their grievances to the formalized judicial process and/or argue for changing that process in the formalized, open "legislature".
For a much better explanation, I highly recommend watching Raph Koster's talk (my previous [2]). Even though it was at GDC and initially talks about VR and MMOs, the talk is not actually about games; it's about the ethical and social responsibilities of creating social spaces, and the may ways we've tried to solve their problems in the past.