[eli]

There's a term for it, but I'm blanking on it.

I believe the idea is that it's for people who are legit trolls. If you let them know, they'll just create another account. But if they think people have just grown tired of their antics, they're more likely to move on to another site.

[pronoiac]

I think you're looking for the term "hellban:" http://www.urbandictionary.com/define.php?term=hellban

[jemfinch]

"Sandboxing" is the term you're looking for.