Remove/Hide User from 'Assigned to' list in Contacts Advanced Search

Hi! I’m looking for a way to restrict/limit or just flat out remove the ‘Administrator’ account in SuiteCRM from the list of users that can be assigned to a Contact.
Ideally there’d be some sort of security group setting I could use.
However, neither the Users or Employees module appear on the list of modules I can control with the default SuiteCRM roles.

In the Security Groups config section, there’s an option to limit the ability to assign users to only users in their group.
After enabling that option, I was able to add all the users except for the Administrator account to my primary security group, effectively solving my issue.

The configuration page is at