public Collection<JID> getOutcasts() {
        return Collections.unmodifiableList(outcasts);
    }
Returns a collection with the current list of outcast users. An outcast user is not allowed
to join the room again. The collection contains the bareJID of the users with outcast
affiliation.

@return a collection with the current list of outcast users.
xmppserver/src/main/java/org/jivesoftware/openfire/muc/MUCRoom.java