Script to set the default value of a Receive_Email field in a Classic form to -3. All other fields named "Receive_Email" are removed to avoid confusion.

default_receive_email_to_unsubscribed.html

<!-- BEGIN Default Receive_Email to unsubscribed. -->
<script type="text/javascript">
    document.addEventListener("DOMContentLoaded", function () {
        // Remove hard-coded Receive_Email hidden inputs.  That will leave the 
        // Receive_Email dropdown that the user can see.
        var a = document.querySelectorAll('input[name=Receive_Email]');
        a.forEach(function(e) {
            e.parentNode.removeChild(e);
        });
        // Set the default value for the Receive Email dropdown.
        var e = document.querySelector('select[name=Receive_Email]');
        if (e != null) {
            e.value = -3;
        }
    });
</script>
<!-- End Default Receive_Email to unsubscribed. -->