Hey richie,
the servers response to LDAP requests can be restricted via LDAP ACLs.
There usage is described in our manual: 3.3. LDAP directory — Univention Corporate Server - Manual for users and administrators
Another point to get some clues might be guides in this forum like How-To: Use LDAP ACLs for Selective Replication or Restrict read access for students.
Best regards
Jan-Luca