Prova questo

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

<html>
<head>
<title>Untitled</title>
</head>
<body>
<input type="button"  value="OK" id="btnOK">
clicca qui
<script language="JavaScript" type="text/javascript">
<!--
document.getElementById("btnOK").disabled = true;
//-->
</script>

</body>
</html>