Preventing Spotlight from Indexing Files & Folders?
Create a file named .metadata_never_index inside a folder.
Rename folders or files so that they end with .noindex.
If you only want to hide files from the GUI, move them to a system folder like ~/Library/.
Move the files to a folder that starts with a period like ~/.fontconfig/. It doesn't remove them from mdfind queries like mdfind 'kMDItemFSName=*' -onlyin ~/.fontconfig/ though.


Related questions:


Exclude files from "central metadata store"
How to add folder to Spotlight "do not index"?
How can I hide the privacy tab from Spotlight.prefpane?