an intranet is a private network so no search bots can crawl your site since they cannot access a private network..
If you make a public facing "intranet" on the public internet network they call that an extranet. Search bots can only crawl and index public facing pages so if you set all of the pages to require users to be in a role, ie uncheck the "All Users" role in View Permissions in page settings and check only a specific role that your users are members of or just check "Authenticated Users" then only authenticated users can access the page therefore no search bot can crawl it.
You can also tell search bots not to crawl using the robots.txt file in the root of the site but that doesn't really prevent them from crawling it, most respectable search bots will obey that but not all of them it is not really a security feature whereas protection by roles will prevent access.
Hope that helps,