We have to unban accounts manually, though users of course can still vouch for their comments in the meantime, which is basically a per-post unban mechanism.
If anyone notices an account that is banned that shouldn't be banned, they're welcome to let us know at [email protected]. We don't want anyone to be banned if they've fixed whatever the original issue was and are abiding by the site guidelines (https://news.ycombinator.com/newsguidelines.html).
On the other hand, I'm surprised how profilic certain people are. They get absolutely no feedback, but will go around trolling people several times a day.