Key: Software\Microsoft\Windows NT\CurrentVersion\Winlogon
Value: 0 disable
The executable for the application is %systemroot%\system32\syncapp.exe. If your users do not have update access to the %systemroot%\system32 folder, renaming the application is a good lockdown method.
If your workstations are installed from a network drive or unattended installation, you can prevent the Briefcase icon from being placed on the desktop by commenting out the syncapp.exe line in layout.inf and txtsetup.sif setup files.