How to hide a forum from Who's Online
 

I have a standard and platinum membership option. I created a forum that is only accessible to platinum members who are over 18 yrs old (I added a custom field for this check since the year-of-birth is optional). I made the forum inactive, so it doesn't appear on the forum list and can only be accessed by the direct URL.

My problem is that I do not want any user to see anyone in that forum. It doesn't show on the "Who's Online" list for the standard members, but it does for all platinum members, regardless of what they indicated for the custom field.

How do I get it to either show only for platinum members who indicated that they are over 18 or instead just show "Viewing Forum" but not indicating the actual forum the user is in?