Same way anything else gets pulled from distribution. You shut down the servers, destroy keys, remove files, etc. Sure, the files may remain in circulation from unofficial sources, but that's not your problem, as long as you're no longer facilitating it.

We are fairly unique for a Linux OS in that we have no central servers, and there are no central keys, and no central entity to even send a C&D to. Everything is a quorum of privately owned hardware, so truly I do not know what mechanism they could even try.