Does anybody know exactly how a Task list gets the list of names it uses
to populate the "Assigned To" dropdown field? It appears to me that it
uses a combination of explicitly authorized site users plus any users in
the Local Admin group for the server. My users don't want to see those
Local Admin names in the dropdown.

Example: If my authorized users are John Doe and Jane Smith, and the
Local Admins are Admin01 and Admin02, then the dropdown values in the
"Assigned To" field end up looking like this:
Admin01
Admin02
Doe, John
Smith, Jane

I found an odd and annoying way to remove the Local Admins, but
hopefully there is a better way (or a way to prevent those names from
appearing in the first place.) I found that if I explicitly added them
to the site via the Manage Users page, and then simply deleted them via
the Manage Users page, they disappeared from the "Assigned To dropdown.

Can anybody shed light on this subject?