try this --
<script type="text/javascript" language="javascript">
function disableBackButton()
{
window.history.forward()
}
disableBackButton();
window.onload=disableBackButton();
window.onpageshow=function(evt) { if(evt.persisted) disableBackButton() }
window.onunload=function() { void(0) }
</script>
then call noBack() function on <body> on the following way:
<body onload="noBack();">
This code will not let the user get BACK after Sign out.