The best thing for you to do in a case like this is to log into your admin area and go to the Groups & Permissions > User Group Permissions area and then click on the Unregistered / Unconfirmed link. For the following options, choose No:
- View member lists
- View member profiles
- View attachments to posts
By doing so, you'll effectively be removing any pages that have become indexed by Google and the other search engines. When accessed after these settings are changed, the pages will return a 403 Forbidden error code. This is exactly what you want. This code will tell the search engines that these pages shouldn't be crawled or included in the index. They'll begin falling out immediately. Don't block these pages in your robots.txt file. That's a very bad move. The best thing to do is remove them fast, which is how I just described.
NOTE: Only follow these instructions if you've also unlinked these pages. You don't want to keep the links to them around your site and have them return 403 responses. That will reduce your crawl budget and your valid page count inside of Google. If unlinked, Googlebot will slowly stop crawling them over time and they'll drop from the index.