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Hrmm. I thought part of the joy of IP blocks was that vandal user accounts can't come in from that IP either. I've used this at least once to ban a logged-in vandal.

If a user on the same IP continues the pattern of vandalism, surely we would ban that user?

Why make life difficult? If we actually know a vandal's IP address, why allow him to use it with all the 30 accounts he's already created?