From what I've seen most of the accounts with a negative reputation were flagged by bots. The @badcontent and @cheetah bots regularly post replies every time a post is made by an account on its blacklist. To the best of my understanding it is possible to earn back a positive reputation but it is probably hard because a negative reputation account often has posts hidden due to low ratings. Negative accounts will still exist on steemit. Even if the negative reputation accounts were deleted the transactions associated with the account will always be in the steem blockchain.