How to get rid of re-appearing Dropbox Finder Toolbar button?
The following procedure works on OS X Yosemite (10.10) and may also work on earlier versions of OS X:


Right click on the toolbar and select the 'Customize Toolbar' menu item
After the toolbar customisation sheet appears you should see that the Dropbox icon immediately follows two 'Flexible Space' items in the toolbar: 
Remove the Dropbox icon by dragging it from the toolbar, and then repeat this action for one of the two flexible space items: 
Close and reopen the finder window




Note: Disabling the Enable Finder integration option in Dropbox's preferences may remove the Dropbox icon from the Finder toolbar, however it also disables sync status icons and Dropbox contextual menu items from appearing.