jQuery

    $(function () {
        $(document).keydown(function (e) {
            return (e.which || e.keyCode) != 116;
        });
    });

    window.onload = function () {
        document.onkeydown = function (e) {
            return (e.which || e.keyCode) != 116;
        };
    }

 onkeydown="return (event.keyCode != 116)"