Hi there Don'tcare.
Thanks for your questions. We'll answer in general terms first to try and explain about the process and bans a bit more and then we'll get on to the thread we think you're referring to.
We take everything on a case by case basis but in general, when we're talking about breaking Talk Guidelines (so things like personal attacks, ageism and generally mean posts) we don't often ban people 'for ever'. We like to give everyone a chance so you'd probably get a couple of deletions and a friendly word first and eventually a temporary suspension of a week or so. If, having done all that, someone still can't post within the rules, we might then look at a longer suspension or even a permanent ban. It really is dependent on what's been going on.
Occasionally, we even allow people back who were banned. We really don't 'want' to ban people, and if we think someone is genuine about posting within the rules and generally is a good sort, we might well allow them to post again. We usually try and get a dialogue going if we think someone is a genuine Mumsnetter even if they've broken the rules, as opposed to being an out and out troll.
Obviously if we think someone is a real 'troll', ie they're just here to cause trouble or are inventing scenarios designed to upset or offend, we'd ban them on sight, and that would be permanent.
In the case of the thread you mention, there seems to have been some confusion about the term 'previously banned poster'. As explained above, you can have been previously banned for breaking talk guidelines repeatedly, not just for being a troll. We had reason to believe that OP was a PBP though it wasn't immediately obvious, for reasons we won't go into here. We had suspicions but we wanted to be sure given the sensitive nature of the thread. In the end, we were pretty confident that this person had been banned before, but we had no reason to disbelieve her story. We didn't feel it was appropriate to allow her to continue to post in this case, but we left the thread to stand because, as far as we could see, it was genuine and we thought it might be helpful both for her (and others in future) to be able to refer back to the advice given.
Hope that gives you a clearer picture anyway. We're sorry if it wasn't clear on the original deletion message.