Have you noticed that there is vbulletin setting that you can disable registering of users who already registered? So if you enable it, they cant reregister by logging out.. Of course they can bypass this by deleting the cookies, but I think this is much more effective than temporary ip ban (which they can bypass by reconnecting to the internet or try 30 minutes later).
I also suggest you checking out HELLBAN hack too, if you havent already did. It can help you too..
Regards,
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