Answer

You can use the ExcludeBlogs configuration directive in mt-config.cgi¹ system to prevent search results from being returned from one or more weblogs.

If the setting already exists in the file, uncomment it², and then indicate the weblog IDs³ you want to exclude from public search results, separated by commas (if the setting does not exist in your file, simply add it to the end of the file on its own line). For example, to exclude weblogs 1 and 2 from search results:

ExcludeBlog 1,2

While this setting will prevent results from the weblogs from being returned via seach forms you've added to your site, it does not prevent someone from actually changing the search URL parameters on their own and discovering results from these weblogs. To prevent that from happening, make sure you're also using the NoOverride configuration directive in your configuration file:

NoOverride IncludeBlogs,ExcludeBlogs

¹ In versions prior to 3.2, the configuration file is named mt.cfg.

² To uncomment a setting means to remove the # from the beginning of the line where it appears.

³ You can determine the ID of a weblog by looking at the URL shown in the address bar of your browser when you are logged into that weblog through Movable Type; look for the blog_id parameter in the URL.