Posts Since Last Reported Abuse
An interesting idea from Kraut and Resnick's terrific book about communities.
"For example, just as some workplaces prominently display a sign showing the number of days since the last workplace injury, a community could display the number of messages since the last reported abuse or the (low) percentage of messages flagged for violating the commmunity's official policies."
It's hard to change the behaviour of a group of people, especially online. Guidelines alone don't work. Social proof helps, as does encouraging normative behaviour.