I believe it is possible to ban them, with a combination of multinational regulation and technical coordination with ISPs. Not easy nor even likely.

ISPs could simply run honeypots that in real time sniff out who is running a blockchain node and block traffic to those IPs. These nodes must broadcast publicly to other nodes to even function, so they can't hide the fact that they are a blockchain node: they are forced to peer with whoever else is... peering in the same direction, pun intended.

I'd love to know what gaps in my knowledge invalidate the above thesis, though.

Is the same not true of torrents?

The difference being that torrents only have ISPs sending out DMCA notices for violators, not shutting off their internet.

Well okay, I mean if you go all out North Korea, I suppose it's possible to ban crypto in your country.

So we're agreed it is technically possible. Technically as in code.