A simple script you can add to your master page or content editor etc. to hide the “Sign in As a Different User” option that is available in the Personal Actions Menu.
<script src="/jQuery/jquery-1.4.1.js" type="text/javascript"></script>
<script type="text/JavaScript">
jQuery(document).ready(function($) {
var objects = document.getElementsByTagName("ie:menuitem");
for (var i = 0; i < objects.length; i++) {
itm = objects[i];
if (('#' + itm.id).indexOf("LoginAsDifferentUser") != -1) {
$('#' + itm.id).remove();
}
}
})
</script>
This is a modification of the original script found here