Restricting Users from iChat Server
When you enable iChat Server, all users with valid accounts in Open Directory (or an alternate directory services architecture) have access to the server. You can restrict access using Mac OS X Server’s service access control lists. Access control lists are configured in Server Admin by the select settings button when the server itself is in the Computers & Services list and then selecting the Access tab (see Figure 2). You can restrict access to any or all services, including iChat Server, and you can deny or allow access to specific users or groups.
Figure 2 Setting service access control lists