How does a Contributor type list get populated ?

I want to make Mat Jones the Owner of a Work Item.
When I select the drop-down list for "Owned By" I can pick Mat Jones, but I can also pick Matt Jones.
Mat Jones (with one 't') is a valid user, he's a member of a few teams and he has an account in the domain's Active Directory.
There is no such person as Matt Jones (with two 't's). At least he's not in Archived Users and he's not in the Active Directory. I'm guessing that once upon a time he was a spelling mistake.

I would like to avoid confusion and remove Matt Jones from my "Owned By" list.
Is that possible ?
How does this Contributor type list get populatted in the first place ?

Thanks    Peter