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Profile names allow for more than ASCII characters. You can find visually similar characters in the vast range of ASCII+Unicode character sets.
Then... change your profile name to be almost the same as another person, change your avatar to be the same as another person... and viola.
To the casual user who might not check the link of the profile page, you are the other person.
For example: marc0m looks close enough to your name that most people wouldn't notice.
Bans on Microcosm are permanent but scoped.
If someone is just causing problems in one forum, I'll favour banning them just from that forum in the first instance.
If someone is causing problems across the site, then the ban will be at the site level.
Impersonating the admin (me) and pretending to post as me is a site level thing (profiles belong to a site and not a forum), so the permanent ban in this case is at the site level.