/**
 * Hide out of stock product button.
 *
 * @param string $html
 * @param WC_Product $product
 * @param array $args
 * @return string
 */
function ace_remove_out_of_stock_product_button( $html, $product, $args ) {
    if ( ! $product->is_in_stock() && ! $product->backorders_allowed() ) {
		return '';
    }

	return $html;
}
add_filter( 'woocommerce_loop_add_to_cart_link', 'ace_remove_out_of_stock_product_button', 10, 3 );