From Coding Horror:

A hellbanned user is invisible to all other users, but crucially, not himself. From their perspective, they are participating normally in the community but nobody ever responds to them. They can no longer disrupt the community because they are effectively a ghost. It's a clever way of enforcing the "don't feed the troll" rule in the community. When nothing they post ever gets a response, a hellbanned user is likely to get bored or frustrated and leave. I believe it, too; if I learned anything from reading The Great Brain as a child, it's that the silent treatment is the cruelest punishment of them all.