<script type="text/javascript" language="javascript">
window.onload = function () {
noBack();
}
window.history.forward();
function noBack() {
window.history.forward();
}
</script>
<body onpageshow="if (event.persisted) noBack();">
in body tag of your page please add this code and in javascript too.
this will prevent user from going to previous page..