You can't because of the way that Joomla manages sessions on the database refreshing it at each page load affecting the chat too.

Even if users can't see the chat you will be able to see them when they refresh pages and navigate the site. Of course they won't use the chat so they won't consume resources but you see them listed in the chat users list.

To strip out them completely from the users list you have to activate the access levels parameter that i said you above, in such way users will be completely filtered by access level too.