Someone in your community does something quite bad, so they get banned from the site for a month. Someone else does something much worse, so they get banned from the site for a year. Finally, someone does something truly heinous, so you think you must try to make them a permanent CommunityExile with a permanent HardBan.
This is a solution, but it is a bad solution.
Instead, ExpandScope, and get the RealWorld authorities involved. If some act is heinous enough to make you consider a lifetime ban, then it should be heinous enough to be against the law, and certainly against the terms and conditions of their internet service provider. Once they're in prison, you can discuss when they might be permitted to return to your community with their parole officer.
That's just passing the buck to another (larger) community, but no more than when we say MeatballIsNotCanonical? or "Wikipedia is not a dictionary". It's canonicalisation again - this time making sure that we create procedures for dealing with serious crimes Wiki:OnceAndOnlyOnce.
If the heinous crime isn't horrible enough to bother the police, consider if it's really as horrible as all that. A related anti-pattern is to DemandApology, which has the potential of transforming a temporary exile into what is, for practical purposes, LifetimeBan.
The above text is PrimarilyPublicDomain