As mentioned above with using nofollow links, blocking a URL in robots.txt won’t work if that resource is linked to from an external site:
Your robots.txt directives can’t prevent references to your URLs from other sites
While Google won’t crawl or index the content blocked by robots.txt, we might still find and index a disallowed URL from other places on the web. As a result, the URL address and, potentially, other publicly available information such as anchor text in links to the site can still appear in Google search results. You can stop your URL from appearing in Google Search results completely by using other URL blocking methods, such as password-protecting the files on your server or using the noindex meta tag or response header.
Again, it seems unlikely this would be an issue here, but it’s something to be aware of.