I have seen that on private forums where they tried to use moderators and things got so restrictive that everyone abandoned the forum. That probably can't happen in a public forum but what can happen is that only the most negative and immature will dominate what would be left of a once good place.
That is basically what happened to the AJ bbs. It's getting better again. There can be a recovery after the moderation is turned on, but it often takes about six months to a year for the quality to recover. Patience is a what?