If you want to logout every time you close your browser, you can do this:

Click "user cp" button at the top of the forum page.
Click "Edit Options" link just below the buttons you just clicked.
or you can go to this link directly:
http://www.bdsmlibrary.com/forums/me...on=editoptions
Choose "no" to automatically login option (the 2nd option)

Will that do?