Форма пример:

<form id="form_req" action="#" method="post">
<input id="aaa" type="checkbox" checked="" />
<input type="submit"  name="submit" id="button_submit" value="Заказать">
</form>

Код js:

$(document).ready(function(){
    $( function() {
        $('#form_req').submit(function() 
            {
                if ($('#aaa').prop("checked")) {
            }
            else 
            {
                return false;
            }
        });
    });
});