Hide Standard Shipping

// Hide ALL shipping options when free shipping is available
add_filter( 'woocommerce_available_shipping_methods', 'hide_all_shipping_when_free_is_available' , 10, 1 );

/**
* Hide ALL Shipping option when free shipping is available
*
* @param array $available_methods
*/
function hide_all_shipping_when_free_is_available( $available_methods ) {

    if( isset( $available_methods['free_shipping'] ) ) :

        // Get Free Shipping array into a new array
        $freeshipping = array();
        $freeshipping = $available_methods['free_shipping'];

        // Empty the $available_methods array
        unset( $available_methods );

        // Add Free Shipping back into $avaialble_methods
        $available_methods = array();
        $available_methods[] = $freeshipping;

    endif;

    return $available_methods;
}