/**
 * Hide all free shipping options when the given shipping class is in the cart
 * Code snippets should be added to the functions.php file of your child theme
 *
 * @return array
 */
function hide_shipping_when_class_is_in_cart( $rates, $package ) {
    // shipping class IDs that need the method removed
    $shipping_classes = array('bulky-items');
    $if_exists = false;

    foreach( $package['contents'] as $key => $values ) {
        if( in_array( $values[ 'data' ]->get_shipping_class(), $shipping_classes ) )
            $if_exists = true;
    }

    if( $if_exists ) {
        foreach( $rates as $key => $option ) {
            if( $option->method_id == 'free_shipping' )
                unset( $rates[ $key ] );
        }
    }

    return $rates;
}
add_filter( 'woocommerce_package_rates', 'hide_shipping_when_class_is_in_cart', 10, 2 );