[rt-users] How do I prevents Requestors from being added to the "Owners" dropdown list?

[Janice Myint]

When a new ticket is created by an outside user, "Requestor", via  
email submission,  the email address of the user is added to the  
"Owners" dropdown list thus making the list very long when the support  
staff tries to assign ticket ownership to themselves.  The privileges  
are set that only the support staff have "OwnTicket" in the  
configuration.  Was wondering if there was another place to look to  
set it so only the support staff appears in the Owner dropdown.

Thanks!
Janice