Prova cosi ma comunque non penso sia molto soddisfacente.

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

<html>
<head>
<title>Untitled</title>
<script language="JavaScript" type="text/javascript">
<!--
var onclickDisabled = false;
function alertDisabled() {
 alert('onclick disabled');
 return null;
} // function alertDisabled() 
//-->
</script>

</head>
<body>
<input type="button" value="clicca" onclick="onclickDisabled == true ? alertDisabled() : alert('pippo')">




<input type="button" value="Abilita" onclick="onclickDisabled = false"> 
<input type="button" value="Disabilita" onclick="onclickDisabled = true"> 

</body>
</html>