To get banned you probably had to do something very terrible. I very much doubt it, unless you contact a moderator apologizing and ask to get unbanned. If not, you have to start all over.
Yeah but does that mean that somebody else can sign up with that username? Like does that un-register and allow somebody else to come along down the road, pick the username and use it.
Like say John gets perma-banned. Does Will who wants to use the name John, can he?
When a user is perma-banned. and his account is deleted. Can another use pick that username. Like does that username become available again?
You'll just get banned again. Unless you are very discreet and don't late people know you are the same person. Some people actually last quite long, but in the end, everyone gets caught and banned again.
All in all it is just a really stupid idea. If you get banned, just go to another site and start over there.