Prototype stopObserving() Method

Syntax:

element.stopObserving(eventName, handler);

Return Value:

Returns an HTML element.

Example:

<html>
<head>
<title>Prototype examples</title>
<script type="text/javascript" 
   src="/javascript/prototype.js">
</script>
<script>

function handler(event){
   alert(Event.element(event).innerHTML);
}

function RegisterFunction()
{

   $('test').observe('click', handler );
   alert("Registering the handler");

}

function UnRegister()
{

   $('test').stopObserving('click', handler);
   alert("Now unregistering the handler");
}

</script>
</head>

<body onload="RegisterFunction();">

  <p id="test">Click me to see the result.</p>
  
  <br />
  <p>Click the button to unregister the handler.</p>
  <input type="button" value="UnReg" onclick="UnRegister();"/>

</body>
</html>

To understand it in better way you can Try it yourself.