Make certain WooCommerce products not-purchasable for non-members

stop-non-pmpro-members-from-buying-woocommerce.php

<?php

/** 
 *  Stop non-members from purchasing products if they do not have an active Paid Memberships Pro Level.
 * Add this code to your PMPro Customizations Plugin - https://www.paidmembershipspro.com/create-a-plugin-for-pmpro-customizations/
 */

function stop_non_pmpro_members_from_buying_woo( $is_purchasable, $product ) {

	// Check if the user has an active membership level.
	if( pmpro_hasMembershipLevel() ) {
		return $is_purchasable;
	}

	// Adjust the restricted categories that require membership level to purchase. Use category-slug.
	$restricted_category_slugs = array(
		'category-1',
		'category-2',
		'category-3'
	);

	if( has_term( $restricted_category_slugs, 'product_cat', $product->id ) ) {
		$is_purchasable = false;
	}

	return $is_purchasable;

}
add_filter( 'woocommerce_is_purchasable', 'stop_non_pmpro_members_from_buying_woo', 10, 2 );

If $is_purchasable comes in as false, may want to just return false. The item may be marked not purchasable for some other reason, and having a pro membership generally wouldn't be a reason to override that.

Thanks @fbarnes, I've updated the gist.