First you should disable “Any-to-Any” from Settings-Options tab. If enabled, this allows all users see all other users.
Go to the Users tab, find and click on that user – a pop-up window will open.
Make sure, the “Allow contacts” in “User info” tab is disabled. If enabled, this user will be able to search and add contacts himself.
Go to the “Contacts list” tab and add the people you want this user to see.
Go to the “Groups” tab and check if this user is a member of any group. If he is a member, you should check if any of these groups has “Any-to-Any” enabled. If enabled, the user will see all group members.
Go to the “One-to-Any relations” tab. If the user has “One-to-Any” for a group, this means he can see all members of this group.