Hello,
This isn't an easy question, and we don't have a good solution to handle it, sadly.
We can't simply delete banned accounts, because it means that the bad person might reuse it, and of course we don't want that.
In particular for e-mails, it means that you can reuse it to create another account, and statistically, someone who have made real abuses is very likely to abuse again if he can.
Leave any door open and you can be sure that it will be found and abused, much sooner and frequently than you think. That's already enough with all the open doors we even don't know that they are there.
That's why banned accounts are basically kept forever.
However, there exists the right of forgiveness.
But when can we safely delete that misused account and be sure that it won't be misused again ? We have already seen banned people who count days and are able to come back precisely when we thought they were definitely gone. So we can't put a simple rule of 1 year or whatever, as we did for simple inactive accounts.
So this isn't something we can fix automatically. I invite you to contact us privately about your specific case.