The simple equation that decides whether you get question banned or not is:
(total questions - negative score questions - closed questions)/total questions >= 0.5
closed questions probably includes deleted questions too. For example, a new user asks a question, and the question gets:
Downvoted, stays at negative score
Deleted before 30 days from posting time
The equation above would not work as:
(total questions - negative score questions - closed questions) < 0.5
Which means question ban. But that is way too early to get banned. It seems that the user has to reach a certain number of questions or rep or time for the above equation to be implied towards the user. I have a small grip on the subject myself. So when does the question ban equation get applied?