Ban by the ip. Then he won't be able to evade it unless he knows what he's doing.
I did. IP addresses change
frequently. It is not enough to ban by IP only so I wanted to keep his account so we could ban by his email, provider host, login name, etc.
If accounts are deleted, then we can't ban by these things anymore so it is much more dangerous to delete an account. We should only delete accounts under RARE circumstances.
Even when users get angry and demand their accounts deleted, almost every time they change their minds, come back and start posting again. This type of activity hurts our numbers and they wind up losing their history and private messages. We should, almost NEVER, delete accounts.
Banned users should DEFINITELY not be deleted because it's a security risk.