You have 2 options here, the first is that you can require site users to use JavaScript, and this is the route that I would suggest.
The other route is a parallel deployment of a non-JavaScript version of your forms that have more robust server-side validation. This is generally a bad idea, and you should have robust validation on the server side anyway.
Normally I'll setup a nested page in my primary view that will be called by AJAX, and can display an error instead if JavaScript is disabled.
For example, with jQuery:
<!DOCTYPE html>
<html>
<head>
...
<script type="text/javascript">
$(function(){
$.ajax({
url: 'my nested view',
success: function(data){
$('#content-wrapper').html(data);
}
});
});
</script>
</head>
<body>
<div id="content-wrapper">
<noscript>
<p>You must turn on Javascript or use a JavaScript-capable
browser to use this site</p>
</noscript>
</div>
</body>
</html>