I have worked with several vB forums and there is a system in them called infractions,
It works by Appling “penalty points” to a users profile; these can be temporary over a certain amount of time, or permanent.
In most of the systems I worked on I made 10 the total, if you reached 10 at any point you would be permanently banned.
The up side to this system is that other users cannot see who has points (it can be hidden) and when the user is banned; if their email address is recorded they cannot use the same email to register again (depending on the settings).