It doesn't hurt to have this on the list, so I'll give a public reply.
1. RIGHT-click on the dropbox item in your toolbar at the bottom of the screen. The Dropbox control panel will appear.
2. In the NE corner there is a little wheel symbol. LEFT-click it, and a menu will appear.
3. Select "Preferences".
4. Under the "General" tab, make sure the "Notify me about .... Synced Files" box is unticked.
5. Click "Apply".
6. Click "OK".