After login you can write the document.ready method of jquery and the inside it just remove the attribute from the body tag.
example:
<script type="text/javascript">
$(document).ready(function() {
$('body').attr( "oncontextmenu");
});
<script>
If dont want to use jQuery then can manage by javascript also:
document.getElementById("body").removeAttribute("oncontextmenu");
Hope it helps :)