If you are using an SEO plugin such as Yoast, it automatically adds all Custom Post Types (and Taxonomies) to the sitemap that is used by Google & other search engines.
You will need to explicitly exclude them from the sitemap e.g in Yoast, this is under the “Post Types”https://wordpress.stackexchange.com/”Taxomonies” tab in the “XML Sitemaps” settings page.
Also don’t forget to remove the links from Google through Webmaster Tools – otherwise you have to wait for Google to re-index your site.