New in Socket.io, How to prevent Socket.io client script hacking

I'm new in Socket.IO, and I've just implemented the tutorial instruction about Socket.IO at http://socket.io/get-started/chat/. It's quite interesting.

But now I have a concern about security. The client code for sending message is:

<script>
  var socket = io();
  $('form').submit(function(){
    socket.emit('chat message', $('#m').val());
    $('#m').val('');
    return false;
  });
  socket.on('chat message', function(msg){
    $('#messages').append($('<li>').text(msg));
  });
</script>

The function call socket.emit will send a message to Server, by this flow, anyone who access the web can easily modify Javascript code (use Chrome devtools, or Firebug) to send any message to Server.

For example, user can add the code lines as following:

<script>
   $(document).load(function() {
       socket.emit('chat message', '1122');
       socket.emit('get_users', null);
       socket.emit('delete_user', 1);        // What ever he wants
   });
</script>

This hack may cause harmful to system.

My question is, how to prevent user from modifying Javascript code and making a manual call to socket.io server, including users who have right to log in web application.

Any help would be great appreciated!