I was able to get this to work essentially the way Dom suggests. I'm often working while tethered, and wanted to stop Dropbox from eating up gigs of metered network bandwidth.
This solution blocks Dropbox.exe on all LAN connections. This works for me because, like many people, I'm always on a wireless connection unless I'm tethering - my only LAN connection is my tethered iPhone. If you actually use a wired LAN connection on a regular basis, you'd need to find a different solution.
- Hit Start (I'm on Windows 8.1) and open "Windows Firewall with Advanced Security." (You can also open Windows Firewall and click Advanced Settings.)
- Click Inbound Rules
- Click New Rule
- Click Program - Rule that controls connections for a program, and click Next
- Click "This program path" and fill in your full Dropbox program path (NOT %APPDATA% or another shortcut - I got hung up briefly here), and click Next
- Select "Block the connection" and click Next
- Select all boxes (Domain/Private/Public) and click Next
- Name your rule (e.g. Dropbox - Inbound Tethering Rule) and click Finish
- Now, right-click the new rule in the list, and click Properties
- Click the Advanced tab in Properties
- Click Interface Types -> Customize
- Select only Local Area Network, and click OK, then OK again.
- Now, repeat steps 2 through 12 for your Outbound rule.
Voila! No more Dropbox activity when you are on a tethered connection. Of course, if you do want Dropbox to sync at times over your metered connection, just disable these two rules in Firewall.