Group Policy mmc will popup. On left panel:
Double-click Computer Configuration to open submenu
Double-click Administrative Templates to open submenu
Double-click System to open submenu
Double-click Turn autoplay off option which will be near the bottom of the list in the right panel.
The default is the Not configured . Set it to Enabled.
I believe that it would be better if you will disable the hardware driver for the USB ports. This will make sure that any kind of portable storage devices including Hard Disk Drive could not be read by the system. Group policy has some problems when it comes to restriction because it might conflict on the Operating System itself. It is better to disabled directly into the Hardware part.