It's been touched upon already, but one of the reasons you see older accounts stick to purgatory rather than banning is that their identity, their personal account that they are known for, is the strongest carrot that we have to dangle in front of someone to force them to behave. Once you take it away, there's nothing left to encourage good behavior.
You absolutely cannot treat all problematic users the same because they aren't the same. I saw mention of Hey earlier. People of that nature can't be treated in the same way as someone like The Admiral because they do things much differently. Hey was stopped by months and months of silent site updates to block each method used to create spam accounts. I have no reason to believe that Admiral would go down that kind of route, which is why he isn't treated the same way.
Having said that, he's been on an extremely slippery slope this year, and managing him via purgatories has gotten rather tedious. Since he's gotten himself suspended yet again, and the "good behavior" window has all but disappeared at this point, he's getting the boot. He almost got it a few months ago on a different suspension but the offense was so minor that a ban just didn't make sense. This time the punishment fits the crime, so that's that.
To those who think it should be a trivial process keeping someone away from the site, I would encourage you to do some research on just how easy it is to access anything you want to on the Internet. We can and certainly do things to slow people down (see: Hey), but if someone wants to be here, they are going to be here. That's all there is to it, and anyone who thinks otherwise is extremely naive.
As for Admiral, I wouldn't start celebrating his banning if I were you. The carrot's gone, and he clearly likes CE. Guess we'll see what kind of person he is.
@SBAllen As a long time poster, I recall a time where troublesome accounts were simply put on KoS. Is this no longer a thing? Why not?