Kashmir Hill writes about How Nextdoor reduced racist posts by 75%. Nextdoor tested a bunch of different approaches:

Some just saw the addition of new language: “Ask yourself: Is what I saw actually suspicious, especially if I take race or ethnicity out of the equation?” Some were asked to say in advance whether they were reporting an actual crime or just “suspicious activity.” Others actually had their posts scanned for mentions of race (based on a list of hundreds of terms Nextdoor came up with) and if a post did mention race, the user got an error message and was asked to submit more information about the person.

This is proof that good design can make a positive difference in the world.