Is there a way to centrally manage Dropbox/centrally disable LAN sync for all Dropbox clients on a Windows LAN?

Solution 1:

One alternative would be to create a rule in Widows Firewall that blocks outbound traffic to the port ranges (or even from the actual program creating the traffic) you are seeing in group policy. That would take the load off the network devices.

Create an Outbound Port Rule on Windows 7, Windows Vista, Windows Server 2008 or Windows Server 2008 R2

This may/should also result in DropBox itself disabling LAN Sync (based on the DropBox LAN Sync help page):

If Dropbox detects a firewall preventing access to your LAN, it will turn off LAN sync in your Dropbox preferences automatically.

Solution 2:

Create a GPO to block the dropbox.exe file from opening.

User Configuration > Policies > Admin Templates > System > Policy > Don't run specified Windows applications.

We use this to also block a lot of the instant messaging apps for a particular department.