constant-contact-signup-disable.php

<?php

function example_disable_newsletter_signup( $form_id ) {
    global $ninja_forms_processing;
    $form_id = $ninja_forms_processing->get_form_ID();
    if ( $form_id == 1 ) {
      $opt_in = $ninja_forms_processing->get_field_value( 23 );
      if ( $opt_in == 'unchecked' ) {
        $ninja_forms_processing->update_form_setting( 'constant_contact_signup_form', false );
      }
    }

}
add_action( 'ninja_forms_pre_process', 'example_disable_newsletter_signup' );