Use this snippet to hide "Free!" price strings from drop-downs and selected product details when the "Per-Item Pricing" option is checked.

woocommerce-composites-hide-free-price-strings.php

<?php
/**
 * Plugin Name: WooCommerce Composite Products - Hide "Free!" Price Strings
 * Plugin URI: https://woocommerce.com/products/composite-products/
 * Description: Hide "Free!" price strings from drop-downs and selected product details when the "Per-Item Pricing" option is checked.
 * Version: 1.0
 * Author: SomewhereWarm
 * Author URI: https://somewherewarm.gr/
 * Developer: Manos Psychogyiopoulos
 *
 * Requires at least: 3.8
 * Tested up to: 5.3
 *
 * Copyright: © 2017-2020 SomewhereWarm SMPC ([email protected]).
 * License: GNU General Public License v3.0
 * License URI: http://www.gnu.org/licenses/gpl-3.0.html
 */
 
// To use this snippet, download this file into your plugins directory and activate it, or copy the code under this line into the functions.php file of your (child) theme.

add_filter( 'woocommerce_free_price_html', 'wc_cp_composited_product_free_price_html', 10, 2 );

function wc_cp_composited_product_free_price_html( $price, $product ) {

	if ( did_action( 'woocommerce_composite_products_apply_product_filters' ) > did_action( 'woocommerce_composite_products_remove_product_filters' ) ) {
		$price = ''; // wc_price( 0.0 );
	}

	return $price;
}

add_filter( 'woocommerce_composited_product_price_string_inner', 'wc_cp_composited_product_free_price_string_inner', 10, 10 );

function wc_cp_composited_product_free_price_string_inner( $price_string_inner, $price_string, $qty_suffix, $suffix, $price, $is_range, $has_multiple, $product_id, $component_id, $composited_product ) {

	if ( $price == 0 ) {
		$price_string_inner = ''; // sprintf( __( '%1$s %2$s %3$s', 'dropdown price followed by per unit suffix and discount suffix', 'woocommerce-composite-products' ), WC_CP()->api->get_composited_item_price_string_price( $price ), $qty_suffix, $suffix );
	}

	return $price_string_inner;
}