Using modern web browsers and enabling "System Tools > Security > General > Use anti xss headers" will force continued HTTPS usage.
Old web browsers can be redirected by enabling MOTD, then adding the following to the end of your MOTD.html file:
<script>
    if (document.location.href.indexOf("http:") > -1) {
        document.location.href = document.location.href.replace(/http:\/\//, "https://");
    }
</script>