I recall somebody asking for a way to log users out of his page. Off course, all anwsers told this is not possible.
However, now comes to mind that a lot of those sex-oriented sites pop up a new window if you leave their page... dunno if this is also working in netscape but is sure does in IE.
My suggestion is -though I have never used it- to use this kind of programming (dunno exactly how they do it, but you can surely find out) to load a page which only logs the user out.
Off course, not all users (including myself) would appreciate this kind of thing, but it seems possible if you realy want it?