You're giving far to much attention to these people which I think is attracting more of their behavior. Now they have an audience and you're just to eager to be entertained. There's a reason most prisons are located away from major cities and the inmates kept away from the public aside from safety.
What you got to ask yourself is this:
Is it better to have them in a "prison" here away from regular members, or with the regular members consistently interrupting the commerce of the website?
Continuously banning him/them didn't seem really to work; so far, how much interruption has there been? At least give it a trial run before automatically presuming it's nonsense.
If you do not wish to partake, don't.
Personally, I think this is a good learning tool. It helps members to identify trolls, trollish behaviour, and perhaps what they do or say that could place them in purgatory, and how to avoid such a place.
Prisons, actual ones, are hardly about rehabilitation (at least in the US), since many of them today are run by private companies, they're more about keeping prisoners because they are their profit margin.
But, the only profit here would be knowledge, and perhaps not rehabilitation but how to successfully combatant such indiscretions.