1. Find the Dropbox icon in your system tray or menu bar. Click it, and a window will pop up. Here's how that looks on Windows:
2. Click the gear at top-right, then click Preferences.
3. This will open the Preferences window. Click the Sync tab, then the Selective Sync button.
4. A new window will open allowing you to choose which files should and shouldn't sync to your computer.
Simply uncheck any files or folders you don't want to be synced to your computer. Dropbox will delete them from your computer and not sync over any future changes to them. These files will still be in your Dropbox, however, meaning you can find them using the web or mobile version of Dropbox.