I want to show you how you can block IPs in your Grizzly server. I pretty sure that you can find lot of reasons why you would want that.
I'll use a list from http://iblocklist.com/ as input for my demo.
What you have to do to close the connection from client that isn't wanted is pretty simple.

    Controller controller = new Controller();
    TCPSelectorHandler tcpSelectorHandler = new TCPSelectorHandler(){
        public SelectableChannel acceptWithoutRegistration(SelectionKey key) throws IOException {
            ServerSocketChannel server = (ServerSocketChannel) key.channel();
            SocketChannel channel = server.accept();
            
            if (isAddressBanned(channel.socket().getInetAddress()))  {
                if(s_logger.isDebugEnabled()){
                  s_logger.debug("This IP [" + channel.socket().getInetAddress() + 
                  "] is banned, the connection will be close");
                }
                closeChannel(channel);
                return null;
            }

            return channel;
        }
    };
    tcpSelectorHandler.setPort(port);
    controller.addSelectorHandler(tcpSelectorHandler);

With that, as soon as a connection is made, Grizzly will validate if the IP is valid and close the connection if required.
I won't show the implementation of "isAddressBanned", because it's not really related to Grizzly, but you can find it in the source code.
You can download the complete source code from here.