From Dropbox official site:
- Click on the Dropbox icon from the system tray.
- Click on the gear icon and select Preferences... from the menu.
- Click the Account tab.
- Click the Selective Sync... button.
A window will appear with a list of all the top level folders in your Dropbox folder. The folders with a check next to them will be synced to your computer. Uncheck any folders that you don't need to sync to your computer's hard drive. When you're done, select OK. Any folders you deselected will be removed from your hard drive, but will still be available through the website and on any computers linked to your Dropbox account.