name thief

A person who 'hacks' access to a username which doesn't belong to him/her without the other person's informed consent and/or any rights exclusive to the original account owner (e.g. an encrypted password).

Where multiple (identical) usernames are allowed, especially on large image channels such as 4chan: a namefag who 'steals' the name of a celebrity or acts as a sockpuppet for an existing user on another website.