I recently posted an updated version  of People and Places that Use The No Asshole Rule.  In that spirit, a group of students in my class Organizational Behavior: An Evidence-Based Approach did a little case study of how a local start-up (with about 150 people now) is sustaining a civilized workplace.  I liked this simple decision-tree as it captures much of the essence of how to enforce the rule — assuming they actually use this rather than just talk about it!