It’s common knowledge that you can stop the contents of a folder from appearing in Spotlight’s search results. Simply open System Preferences, click Spotlight, select Privacy, then drag and drop the folder onto the program window. This also works with individual files.
However, there’s a quicker way to stop Spotlight indexing the contents of a folder that you’ve created, or that already exists on your hard disk: add .noindex to the end of the folder’s name.
For example, if the folder is called bob party pictures, rename it to bob party pictures.noindex. If the files have already been indexed, making this change will cause Spotlight to forget about them so they don’t appear in search results. If you’re creating the folder from scratch then anything you place there, such as downloaded files, won’t be indexed by Spotlight, so won’t appear in search results.
To undo, and have Spotlight index the files, simply remove the .noindex extension on the folder.