Unlike many here, I have some degree of confidence that users who are close to a ban can actually learn what they're doing wrong and improve. I also have confidence that problematic users are not just heuristic engines designed to post questions which marginally evade the filter. I think most of them would actually learn how to post good questions, if they were pushed to.
The question quality filter should become stricter as users move closer to a ban. This will force those users to post better questions, and hopefully help them learn. If they figure out what they should be doing, then they will, perhaps, avoid a ban entirely.
I'm not saying we should change the ban threshold at all. I think the banning mechanism is actually tuned fairly well now. However, we could be utilizing the question quality filter to push users in the right direction.