Prevent non-members from adding member products to their cart with WooCommerce and PMPro.

my_keep_members_only_products_out_of_carts.php

<?php
/**
 * Prevent non-members from adding member products to their cart.
 * This requires the PMPro CPT Add On be installed and
 * each product should be checked for which levels are required.
 */
function my_keep_members_only_products_out_of_carts( $is_purchasable, $product ) {
	// Not purchasable for some other reason.
	if( ! $is_purchasable ) {
		return $is_purchasable;
	}
	
	// Make sure PMPro is active.
	if ( ! function_exists( 'pmpro_has_membership_access' ) ) {
		return $is_purchasable;
	}
	
	$product_id     = $product->get_id();
	$has_access = pmpro_has_membership_access( $product_id );	
	$is_purchasable = $has_access;
	
	if ( false === $has_access ) {
		// Add the warning message for the single product summary page(s)
		add_action( 'woocommerce_single_product_summary', 'my_pmprowoo_purchase_disabled' );
	}
	
	return $is_purchasable;
}
add_filter( 'woocommerce_is_purchasable', 'my_keep_members_only_products_out_of_carts', 10, 2 );

/**
 * Info message when attempting to add non-member product to your cart.
 */
function my_pmprowoo_purchase_disabled() {	
	?>
    <div class="woocommerce">
        <div class="woocommerce-info wc-nonpurchasable-message">
			This product is for members only.
        </div>
    </div>
	<?php
}