There are moderators for egregious indisputable violations, just like there are police for lawbreaking. But there are also murky or less clear-cut cases that nevertheless clearly degrade the quality of the discussion over time, just like there are lots of mean things people do to each other in the real world that appropriately are not punished by the police. The way these are dealt with is by telling the person they shouldn't do what they've done, and by arguing to others that it shouldn't be supported.