How to prevent write access in ou=system as anonymous user?

[Ado Dao]

Hi

I have implemented a custom partition with Java. The partition intercepts the 
LDAP search requests and redirects to an internal logic. Currently, this runs 
in anonymous mode. With LDAP client it is possible to add entries in ou=system. 
e.g. cn=test, ou=system. How can I prevent that this is possible in anonymous 
mode?

thx
Ado