I've got 100K on 2 sites - MSE and Super User. That literally does not mean that I know anything about programming. As an experienced SE user, I have no illusions that having high rep on one site means that I am any good in another.
Just letting people keep asking bad questions is a bad idea.
We agree there
However, locking someone out of asking questions permanently maybe too harsh.
We don't, you get an opportunity to ask questions every 6 months and if you can contribute positively in other ways, and demonstrate you know the site - with good answers and suggested edits, you might be able to get out.
What about if the person just need to get used to it? What about if his questions in other part of stackexchange is fine.
My reputation shows I know the 'basics' of SE. It does not reflect any competency beyond that or competence of some sort in the subject matter of the specific site.
I suggest that users can have their sin purge in one stackexchange say at the cost of 500-1000 points in other stackexchange. That's hefty price. They'll get better.
That's.... maybe a good month's worth of rep for me. And I could pretty much mint get out of question ban cards anywhere. The end result of this though is people will go site shopping mainly for reputation, get frustrated, and also frustrate users there.