It's not possible on IPv4 either, your IP is one of the easiest thing for you to change...

Browser/device finger printing is probably the best option at the moment, but you'll get plenty of false positives amongst privacy aware users who block fingerprinting.

I think the rough IPv6 equivalent of a single IPv4 ban would be to ban a /64 subnet. This bans all users in that subnet which is the same as banning an IPv4 subnet hiding behind a single NAT address.

I'd say the same way you ban them with IPv4. IP bans are trivial to circumvent, so those who won't bother to/can't circumvent them with IPv4, also won't with IPv6.

It's the same as in IPv4.

If the user has a static IP address, in IPv4 you ban a /32 (i.e., one address, his own). In IPv6 you'd have to ban the entire range allocated to the user, usually a /64. (edited)

If the user has a dynamic IP address, well, you're on your own. :D

The alternative is to let them post and just hide their stuff from other people; see https://blog.codinghorror.com/suspension-ban-or-hellban/ for example.

HN has a system like that, but it fails because some people can still see the 'hidden' / dead posts, and for some reason they then go and point out "your comment shows up as [dead]", which defeats the purpose.

Just ban people without letting them know.