It may also be possible to split your forums to different sites / domains if you reach the thresold.
Anyhow I can't see how this will play out in real life and I hope our president will step in and prevent it from happening at all.
But I am 100% sure, our current government would never ever grasp the concept of a federated social network and probably just try to ban it.
I'd probably keep the content but orphan it and kill the underlying user and profile.