That's not entirely accurate.

For email see https://news.ycombinator.com/item?id=32715437

For website see https://news.ycombinator.com/item?id=32706673

Neither one of those are counterexamples. One guy decided to give up solving his deliverability problems (problems I don’t have after self-hosting my email for 20 years). Nobody banned him. The second example is cloudflare banning a site. I don’t (and wouldn’t) use cloudflare, and even in that case, nobody banned anybody from his own website—just from cloudfare.

Regarding email, it's still possible for practically every other mailserver on the Internet to block messages from you, effectively banning you. That's what happened to OP I believe.

On the second one you might want to look further into that case... people have been frantically going after every possible company that does any business with them trying to get them off the internet, whether that's hosting providers, DDoS protection services, IP allocation providers, upstream ISPs, nameservers, domain registrars, etc.

They can, it just takes extra steps and so probably won’t happen as nobody is likely to run a coordinated campaign to do it.

How would it work? I mean, I can imagine framing me for a felony and getting me banned from internet access as part of my sentence, but are there less extreme procedures?

EDIT: Maybe breaking into my email server and sending out spam to poison my IP. That’s conceivable, and would partially ban me from sending out email.

DDOS, reporting your server/domain for TOS violations, etc.

As long as nobody cares enough to do it, it probably won’t happen.

Just never piss off the wrong person.

That said, how much costs ddos?