To view current browsing sessions of the authenticated users, navigate to Squid / Auth / Captive Portal node in Admin UI, tab Sessions as shown on the following screenshot.
Select the session to remove and click the Remove Selected button on the right. The session will then be removed from the database and any new connection from the user will then be redirected to Captive Portal page.
To let the user logout manually at any time, from the user machine, navigate to
https://your_proxy_name_or_ip/portal/logout/ and click the Log Out button.