I have a unknown source attached to my account. How do I prove my identity in person?

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I have a unknown source attached to my account. Either Windows's or IOS system. My device was stolen and my identity has been compromised. The hacker put Norton in my name before me. Now I can't get Norton to remove the false account. How do I prove my identity in person?

[Edit: Clarified the subject]

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I take it that you did contact Norton Support,   www.norton.com/contactcs  ,   and explained the situation? If the first line agent cannot help ask for a supervisor.