Unset Specific Shipping Methods if Free Shipping is Available

functions.php

<?php 
// =========================================================================
// Unset Specific Shipping Method if Free Shipping is Available 
// =========================================================================
function unset_shipping_when_free_is_available_in_zone( $rates, $package ) {
    // Only unset rates if free_shipping is available
    if ( isset( $rates['free_shipping:8'] ) ) {
        unset( $rates['flat_rate:1'] );
    }     
    return $rates;
}
add_filter( 'woocommerce_package_rates', 'unset_shipping_when_free_is_available_in_zone', 9999, 2 );