Ahh, that's definitely a good explanation of why it wouldn't work. I know Dreamwidth is the only place that's ever gone out of its way to explain why it couldn't help me or friends of mine in situations where someone has broken into a journal, and IP addresses were used as the excuse. The person lived in another state, but they couldn't help us because they said they couldn't prove that we hadn't posted from that address.

Would an alternative to be to ban by email if you knew the email?

(In addition, I've had miscommunication on Livejournal where they banned me and my entire household by IP from Livejournal before and it was almost impossible to contact them since we couldn't visit the site at all to figure out the support email.)