How to override public method within Javascript instance

As a newbie in Javascript I have a question regarding public method overriden. It is about Dojo copyState method, check the doc here.

By default dojo allows copy nodes, but I would like to turn it off, so that i create following Dojo.Dnd object:

var order_list = dojo.dnd.Source("order_list");

then how to override the public method?

dojo.byId("order_list").copyState:function( keyPressed, self ){return false;};

seems does not work, FireBug gives:

SyntaxError: missing ; before statement

dojo.byId("order_list").copyState:function( keyPressed, self ){return false;};
---------------------------------|