You know, there's one solution that hasn't been brought up in relation to impersonation that'd work the best because we're already doing it.

Moderate servers so that people intentionally impersonating other users are not allowed, and then block other servers that actively refuse to do this.

@b that's better than nothing but it's not very reliable
I guess that's kind of what we do with email, with varying levels of diligence/success
@b How do you tell whether to shoot the one with the goatee or the one without
@b Sounds like a lot of work for moderators but if there were admin tools to make it quick and clear...