I understand the idea of a question ban - if you keep making bad questions you can't do so anymore. But what's to stop someone from simply creating a new account and continuing to do so?
The question escalates to the point, what can the SE do with wrongdoings by the same person, from continuously re-created (and banned) accounts.
The answer is, that doing this "correctly", the SE has no way to avoid the behavior.
However, doing this "correctly", is harder as it seems on the first spot. And the wrongdoers have no way to know, exactly what is the SE doing to detect them.
What the SE can detect:
- IP (or network) of the wrongdoers (it can be falsified, but not easy)
- browser version string and other fingerprinting (most parameters can be easily falsified, but not all, and we have no way to know, exactly which ones are watched)
- if the attacker, because from this point they are attackers, is coming from the tor network
- wording / typical grammatical mistakes of the wrongdoers (the SE probably doesn't utilize any algorithm for that, but the mods probably can detect the easier cases with experience)
On the meta SE, there are sometimes mentioning from previous or current "fights". The main threat of the SE system is the spamming and the voting circles, and not the LQ questions. Of course the the exact details are not public.
As far I know, the main strategy of the SE in the case of the LQ posts is to enforce them to post also useful content, and then filter out the rest.
There are technical measures in place to make this difficult. As posted on Meta Stack Exchange,
Can I simply create a new account?
No. The automatic ban is at a lower level than account.
There isn't much of any information available about what "lower level than account" means. The details are intentionally kept secret by SE, to make it more difficult for people to game the system. However, it seems likely that the system takes into account IP address among other factors.
Besides the technical measures, there is also some amount of human oversight. As Jyrki Lahtonen posted in a comment,
Moderators have tools for detecting that. They are somewhat ... secret. Actually, such askers are often too lazy, and keep reasking the same (or at least similar) questions. Many eagle-eyed regular users can spot those even without the tools diamond mods have. They then summon a moderator to the scene.