How to prevent user to add friend?

Hi everybody!

I have just set up Openfire and Spark. I very happy with them. I’m using it as live support software. Only one problem, i don’t want users can chat together. I want they only see 4 administrators only. I’m successfully to distribute 4 administrator accounts to every user. But they still can add new friends.

How can i prevent them to add new friend, only chat with 4 default admin account?

Thank in advance!