My uncle is in charge of policing Millwall home games. He says they've got a lot better in recent years, due to the club focusing more on being a family club and many of the fans (including the old hooligan element) growing embarrassed about their reputation. People are still kicked out at the Den but with very minimal fuss due to how the police approach the situations, even if an officer see's you do something wrong they won't kick you out until the cameras have backed up them, so most people now know that if the coppers tell you to leave then they've got you and you can't deny it, so just walk.
He also tells some good stories about how they "sort each other out" for example in Abdou's first season there someone shouted "oi you black c*** sort yourself out", to which an known "treatment" member replied with "stop the fa**ing swearing, there's fa**ing kids around, before proceeding to knock him out