Duncan, I thought LiveID links to an email add? but i guess they are also creating throwaway emails?
does LiveID verify the email? -- been a while I created my stuff back when it was still called Passport 
between junk / fake / throwaway emails and dynamic IP / spoofed IP it makes it hard to block only the "bad guys"
sucks.... wish I had some crazy great idea on how to deter them..... other than making our site require some kind of approval before first post.
what about that?
like a new user can not post untill approved.
require some email reply as one step
possibly let them post but hide the first 24 hours of posts "pending user verification"
possibly just a way to slow them down?
so that if they sign up and post junk no one sees it on the pages, someone can see what they post and allow/ban then.
granted this makes for a hassle for the staff to check on new users but it might scare off the spam-posters
or at least make them have to wait to get any results.... that hold might send them elsewhere.