In most countries they can block access to know relays and also require AppStores to remove nostr apps in that country.

The US shuts things down by seizing domain names. China and a few other countries can do deep packet inspection to block kinds of traffic, it wouldn’t be too hard to see that some traffic is nostr, especially because a MITM attack might be invisible if the nostr client doesn’t alert the user.

There are nostr relays on the tor network and that is A LOT harder to block. As far as I know there is no union router service that loads a nostr web app configured to use tor relays. That’s not very hard to do actually.

Blocking nostr is hard. Making it dificult for non-technical users to get access to nostr is much easier.