Excluding users in child domain

Hello Everyone,

We currently have a domain (contoso.local) which our SharePoint environment lives and operates.  We also have a child domain for students (contosocats.contoso.local) which SharePoint is not used in this domain.  However, while adding users to SharePoint Groups it is finding users in contoso.local as well as contosocats.contoso.local.  How would I exclude SharePoint from finding the user accounts in my child domain.

What have done so far is in Central Admin, unchecked the child domain from the User Profile Sync Connection Settings.  It still find users from the child domain.  I have also created an exclusion in the Connection Filters to exclude a mail attribute that all the child domain users have.  However I am still seeing users come up when adding people to groups.

Anything else I should be looking for?

Edit: I have also ran a user profile sync after making these changes.

Thanks for any information,

Ian...