0 Block Dropbox with GPO.

Computer Configuration\Policies\Windows Settings\Software Restriction Policies\Additional Rules\Path Rules: (New Path Rule)


For older versions of Dropbox : 
%AppData%\Dropbox\bin\*.exe (Disallowed)
For new versions of DropBox : 
%AppData%\Roaming\Dropbox\bin\*.exe (Disallowed)

To update GPO on target PC or just check on your own PC : 

RUN CMD as Administrator : gpupdate /force