Disable Free Shipping on a product by product basis in WooCommerce. Used in this post: http://speakinginbytes.com/2013/11/disable-free-shipping-woocommerce/

disable-free-shipping-per-product-woocommerce.php

<?php
/**
 * Disable free shipping for select products
 *
 * @param bool $is_available
 */
function my_free_shipping( $is_available ) {
	global $woocommerce;

	// set the product ids that are ineligible
	$ineligible = array( '14009', '14031' );

	// get cart contents
	$cart_items = $woocommerce->cart->get_cart();
	
	// loop through the items looking for one in the ineligible array
	foreach ( $cart_items as $key => $item ) {
		if( in_array( $item['product_id'], $ineligible ) ) {
			return false;
		}
	}

	// nothing found return the default value
	return $is_available;
}
add_filter( 'woocommerce_shipping_free_shipping_is_available', 'my_free_shipping', 20 );