The reason for hellbanning is https://news.ycombinator.com/item?id=5413912

"Adria Richards is an evil cunt.

SendGrid is a shit company that I would never want to work for."

Not exactly stellar material.

This is why bans should start with a warning - hellbanning may be efficient from an administration perspective, but among other issues, without the possibility of appeal it's fundamentally unfair and prone to mistakes. (The ability of users to appeal after noticing something you're explicitly trying to hide from them isn't really adequate.)

But yes, this ban is probably deserved.