Exclude directory from Unity Search

Actually I think this is a built-in feature. It's a common thing to need to exclude directories from your search. Here's how to configure it:

Open the system settings
Select "Security & Privacy" Go to the tab named "Files & Applications"

You should see a box named "Exclude", use the "+" and "-" buttons to update this. Anything NOT excluded will be used in the search.

Note that you will also need to "Clear Usage Data" from the "Files & Applications" dialog in order to remove previously browsed files and folders from the Dash Search. By default Dash Search will include your historical file usage even if you have explicitly excluded these files.

You can make a directory/file hidden. I. believe that unity does not search into hidden files and folders.

To hide a file, add a dot. before it. I.e to hide a folder named documents, right click it, click on rename, and rename it as .documents