Cacher is the code snippet organizer for pro developers

xattr -w com.dropbox.ignored 1 

Dropbox Ignore Feature in Beta
Dropbox (via Hacker News):

You can set a file or folder to be “ignored” by Dropbox. This allows you to organize files and folders in the Dropbox folder on your computer without storing them on dropbox.com or on the Dropbox server at all.

STEPS

- Open the Terminal application on your computer.
- Go to the root file path.

TYPE:
xattr -w com.dropbox.ignored 1 <<ADD FILE / FOLDER PATH>>

EXAMPLE: 
xattr -w com.dropbox.ignored 1 /Users/sjw/Dropbox/CODE/CODE_DEV/_PRACTICE/BradTraversy/react_hooks_todo/node_modules


Type the location of the file next to that.
You can also drag and drop the file or folder that you want to ignore from your file browser into the Terminal and it will populate with the location of the file.
It should look something like this:

xattr -w com.dropbox.ignored 1 /Users/yourname/Dropbox\ \(Personal\)/YourFileName.pdf