I'm assuming you are using LS 2.0.
It's not easy to disable a single item of a sortable list (ranking) but I think it may be more user-friendly to remove the item entirely anyway.
Try adding this to the source of the question. The script will remove the ranking item that contains the exact text as the LASTNAME attribute.
<script type="text/javascript" charset="utf-8">
$(document).ready(function() {
var lastName = '{TOKEN:LASTNAME}';
$('.dragDropTable li').filter(function() {
return $(this).text() == lastName;
}).remove();
});
</script>
