The problem isn't topics. The problem is posters. If it was even tangentially related to tech (a new start-up community in a ghetto?), someone could post a well thought out and reasoned statement arguing against ethnic diversity. I would be amazed if they did, but anything is possible. Just like people can post incredibly shortsighted and emotional crap about what text editor to use.
So rather than make a blanket (and half-assed, at that) statement on a topic being banned (will be interesting to see what gets removed and what/who YC are financing or partnering with in the near future...), remove the negative posts while leaving the good.