Using required checkbox validation to prevent form submission

I've got some code that checks if at least one checkbox is selected and then submits the form (MailChimp). My problem is I don't know how to stop the MailChimp submission if it catches that no checkboxes are selected. Right now it shows the alert and then submits the form anyway ... little help.

<form onsubmit="valthis()" action="//mailchimp form" method="post" id="mc-embedded-subscribe-form" name="mc-embedded-subscribe-form" class="validate" target="_blank" novalidate>

// form html

        <script>function valthis() {
var checkBoxes = document.getElementsByClassName( 'myCheckBox' );
var isChecked = false;
    for (var i = 0; i < checkBoxes.length; i++) {
        if ( checkBoxes[i].checked ) {
            isChecked = true;
        };
    };
    if ( isChecked ) {
        alert( 'At least one checkbox is NOT checked!' );
        }   
}</script>

<input type="submit" value="Subscribe to notifications" name="subscribe" id="mc-embedded-subscribe" class="button">