How would a lockdown work to demotivate a spammer, without inconveniencing the other users to such a degree that they'll also leave?
Besides, I think the whole 'having to make a new account because the other one got banned' thing we've got now is just as much a hassle for trolls, and doesn't impact the rest of the userbase. But that again means a mod has to be online to wield the banhammer, just like with a site lockdown.
In the end, there unfortunately is no magic 'make trolls gk away' button. But we've managed to survive Ryder and TSR, and we'll probably survive this one as well.