Do you hold patents on key technology in implementing the network? Is that even possible with a open source project?
I'm not sure if they hold any patents, but its certainly possible to hold patents and open source the code. This will create a legal barrier that may prevent a fork.
However this is not very practical for a few reasons:
1 - A fork might be created in a another country outside their jurisdiction
2 - Many workarounds can be made in a medium such as code, For example game engine designer John Carmack made changes to code that performed a very similar operation but removed the potential for litigation.