Required Checkbox Field Has No Effect
You can add your form validation handler (hook_form_alter):

$form['#validate'] = array('delete_validation_handler') + $form['#validate'];


Your validation handler:

function delete_validation_handler($form, $form_state){
    if(!$form_state['values']['confirm_delete'])
        form_set_error('confirm_delete', t('Message about to confirm it first'))
}