Back the folder up, always GOOD!
copy the subfolder that you want to not sync out of dropbox.
Let that sync
Make a folder with the same exact name in the same exact place EXACT case!!
RighClick the DROPBOX ICON.
Click the GEAR
Click Selective Sync
Use Selective Sync to stop syncing that folder. It will remove the folder.
Copy your original folder back in. It'll get a gray minus icon, and is no longer syncing. FREEING UP DISKSPACE on dropbox!
EASY! AS CAKE!!!