By default, all methods in a zone are presented to the customer. If you have conditional free shipping and want it to be the only shown method if it’s available, you can use the following snippet to hide other methods:

<?php
/**
 * Hide shipping rates when free shipping is available.
 * Updated to support WooCommerce 2.6 Shipping Zones.
 *
 * @param array $rates Array of rates found for the package.
 * @return array
 */
function my_hide_shipping_when_free_is_available( $rates ) {
    $free = array();

    foreach ( $rates as $rate_id => $rate ) {
        if ( 'free_shipping' === $rate->method_id ) {
            $free[ $rate_id ] = $rate;
            break;
        }
    }

    return ! empty( $free ) ? $free : $rates;
}

add_filter( 'woocommerce_package_rates', 'my_hide_shipping_when_free_is_available', 100 );