How to stop keyboard working.

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ASP.NET

Javascript

jQuery

I've made an online examination portal in asp.net .In this portal user can insert characters only by virtual keyboard by mouse, I have applied a script which is working fine on several machine. Keyboard is totally stop working, but in school labs some system after long press any key and then back space 2 times user able to use keyboard.How to block keyboard working specially alt+tab and windows key, I'm not able to figure out what problem is occurring same code working fine on some and some not. Thanks in advance script is here,

<script>
            document.onkeydown = function (ev) {
                var key;
                ev = ev || event;
                key = ev.keyCode;
                ev.cancelBubble = true;
                ev.returnValue = false;
            }
</script>