Kane is right - blocking the whole IP-range would block the whole operator, which has around ~35% of local market share.
I am not sure what is the situation globally, but on local market ever since the 4G LTE networks came, the operators have strongly favored cellular connections over land lines. This makes sense as LTE gives you decent 100Mbps speed and pings below 20ms, and most likely is cheaper to maintain for the operator and even even more robust than landlines. The connection has a NAT at the operators end (they charge extra for a public IP), and the IP is really changing frequently. This is very different from ADSL and other cables, where IP mostly remained unchanged for months, unless you changed your MAC.
I have no idea how will things work with 5G and IPv6, but the future is most likely wireless (or cable-less).
I like @mpalmer’s idea of tagging users at the point of registration. I don’t quite agree with the privacy concerns, as previously admins used to be able to identify users with the IP-address with similar probability. And cookie does not actually tag the person, but the specific browser in the specific device – even if a dupe id-tag is found, one cannot be automatically suspended, as it could be a shared device.
As the IP-address has lost it’s significance in user identification, I would love to see the cookie tag approach in the future of Discourse. It’s not rock solid, but it adds a layer of security.
Our trolls are not typically genius cyber criminals. They are typically premature people with an odd idea of having fun. This is sports forums, so think New Jersey fans trolling NY Rangers fans. Another archetype is that someone goes too emotional, and gets a short suspension, after which he goes to berserker mode and comes to rage with a new account. IP-address they may be aware of, as it is old news, but quite certainly a cookie would catch them by surprise.
Edit: Another thing that is making the IP-address less useful is the emerging consumer VPN services. For example the F-Secure Freedome seems to be gaining some popularity and Opera browser now ships with VPN. This seems to be a big trend now.