Admin user can speak to all other users but users can only speak to an admin user.
You can set up channels in the web console that are 'dynamic'. This means that if you put an admin user and all other users on this channel, it would not automatically populate all users contact lists with all user contact names on the channel. This would allow everyone to talk on the channel, but not one-on-one outside the channel.
You can then configure the admins' usernames with privileges of 'direct contact' with everyone else. This way they can talk to all users one-on-one, but the users cannot talk to each other one-on-one.