[Erwin]

If you are absolutely sure you have closed your forum, then your forum is closed.

When a member tries to access any of the pages of the forum, Who's Online will say that they are looking at that page, but what they are really seeing is the "No Permission" page. So if I was you I wouldn't worry (unless you are certain that your site has been hacked). Even with the forum closed, vB still tells you the number of people who are trying to access each page.

If you're worried, open another window, log out, log in as a registered member on a test account and try any of the pages - see on Who's Online that vB tells you that a member is looking at a page when in reality the member is getting the "No Permission" page.