code: public void signOff(String key, WebSocketConnection connection) {
          ConcurrentMap<WebSocketConnection, WebSocketConnection> connections = registry.get(key);
          if (null == connections) {
              return;
          }
          connections.remove(connection);
      }
docs: Detach a connection from a key.
  
  @param key
  the key
  @param connection
  the connection