try this bit of javascript:
<form action="page2.php" name="myform" method="post" onsubmit="javascript:kill_form()">
<input type="text" name="fname" />
<input type="text" name="lname" />
<input type="submit" name="submit" />
</form>
<script type="text/javascript" language="javascript">
<!--
function kill_form() {
document.myform.submit.disabled=true;
}
//-->
</script>