I don't see how individuals can be permanently banned.
I help run and moderate a small hobby forum. Over the years we have had a number of prolific trolls who come back time and again.
Some are pretty rubbish at tech and you can catch them out because they use the same IPs but most just use VPNs and change their IPs constantly. They usually get banned in the end anyway because of their behaviour but you can't stop them reregistering and coming back often with multiple accounts.
I suppose the issue for people like DRRADFEM is that if you do end up on a list of permanently banned users you can no longer use the site and be upfront about who you are.
This would be an issue for anyone who tweets from a position of having credentials in a specific area so for example Hadley Freeman has a twitter platform as a well known journalist which is how she gains followers and a significant voice. If she was perma-banned nothing can stop her coming back but she could only do it anonymously which reduces her status to pretty much nothing.
Not that I think twitter are going to be banning prominent blue tick users for the most part but it would affect individuals who have built up significant followings but are not famous or public figures in any official capacity.
It makes me uneasy because it has the potential to seriously interfere with political organisation depending on the bias of the companies that own the platform and any group that may have influence with them.