Fans of O.A.R. were so rowdy at Jones Beach Theater three years ago that the rock group has been banned from the premises ever since. More than 20 inebriated minors were carted to local hospitals that night—victims of group chugs, beer pong games and other parking-lot diversions.

In response to that incident—and a number of others like it —O.A.R. recently adopted a fan code of conduct, dubbed the "Don't be that guy (or girl)" policy. Among other no-no's, the code demands that fans refrain from fighting, doing illegal drugs,...