By default WooCommerce will still allow a free shipping method to be selected if the coupon code is filled in and active in the shopping cart. The following code can be added to your child’s functions.php page to hook in and hide any free shipping option if a coupon code is used.
add_filter( 'woocommerce_shipping_packages', function( $packages ) {
$applied_coupons = WC()->session->get( 'applied_coupons', array() );
if ( ! empty( $applied_coupons ) ) {
$free_shipping_id = 'free_shipping:2';
unset($packages[0]['rates'][ $free_shipping_id ]);
}
return $packages;
} );
Simply change the $free_shipping_id in the above code to the ID of your free shipping option. You may find your ID by going to WooCommerce > Settings > Shipping Options and then click on your Free Shipping option. The post ID will be in the URL of the page that displays the Free Shipping details/settings.
i.e www.yourdomain.com/wp-admin/post.php?post=40107&action=edit
Where 40107 is the shipping ID in this example. Your shipping ID will be different.