Prova cosi.....

codice:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">

<html>
<head>
<title>Untitled</title>
<script language="JavaScript" type="text/javascript">
<!--
function disabilita() {
 for (i=0;i<document.body.children.length;i++) {
  var curObj = document.body.children[i];
	if (curObj.tagName == 'INPUT' && curObj.type == 'button') {
	 curObj.disabled = true;
	} //	if (curObj.tagName == 'INPUT' && curObj.type = 'button')  
 } //  for (i=0;i<document.body.children.length;i++)
} // function disabilita()
//-->
</script>

</head>
<body>
<input type="button" value="clicca">

<input type="button" value="clicca">

<input type="button" value="clicca">

<input type="button" value="clicca">

<input type="button" value="clicca">

<input type="button" value="clicca">

<input type="button" value="clicca">

<input type="button" value="clicca">

<input type="button" value="clicca">



<input type="button" value="disabilita" onclick="disabilita()">
</body>
</html>