Social norms as a regulatory mechanism often carry more weight than formal law--particularly in contexts when legal rules are gray. In online creative communities that focus on remix, community members must navigate copyright complexities regarding how they are permitted to re-use existing content. This paper focuses on one such community--transformative fandom--where strong social norms regulate behavior beyond copyright law. We conducted interviews with fan creators about their "unwritten rules" surrounding copying and remix and identified highly consistent social norms that have been remarkably effective in policing this community. In examining how these norms have formed over time, and how they are enforced, we conclude that the effectiveness of norms in encouraging cooperative behavior is due in part to a strong sense of social identity within the community. Furthermore, our findings suggest the benefits of creating formal rules within a community that support existing norms, rather than imposing rules from external sources.