Google won't be able to index pages it cannot see, so visibility settings on products and categories should ordinarily suffice. However, if for some reason you have additional pages that you wish to keep available to shoppers, but away from Google, we offer a robots.txt addendum in Settings / Policies, Addenda and Disclaimers / Edit / Robots.txt Addendum which allows you to add custom commands to robots.txt (http://DomainAlias/robots.txt), either above or below the references to your sitemap(s).
A noindex or disallow command can be placed here for any page you wish to omit. Note that your store MUST be using a domain alias for this to work.
Here are a few valuable links that describe which syntax might work best for you:
Please Note: The Robots.txt Addendum is an advanced feature that should be used only by those familiar with robots.txt commands and their impacts. Improper use of this field may significantly harm your search engine rankings.