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 hn@ycombinator.com. 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).