code: public void signOff(String key, Collection<WebSocketConnection> connections) {
          if (connections.isEmpty()) {
              return;
          }
          ConcurrentMap<WebSocketConnection, WebSocketConnection> bag = ensureConnectionList(key);
          bag.keySet().removeAll(connections);
      }
docs: Sign off a group of connections from the registry by key
  
  @param key
  the key
  @param connections
  a collection of websocket connections