In SharePoint, the content in each list can be configured to be included or excluded from being indexed in search crawls.
Through the SharePoint UI, this option is called "Allow items from this list to appear in search results?" and is located in the Advanced List Settings:
Through the SharePoint API, this option is exposed as a property called "NoCrawl" (for the list named "Hidden From Search List"):
SPSite site = new SPSite("http://siteurl");
SPWeb web = site.RootWeb;
SPList list = web.Lists["Hidden From Search List"];
list.NoCrawl = false;
list.Update();
After this property has been updated, the next time SharePoint Search crawls, it would skip indexing the list with a NoCrawl value of false.