Is there a hack which forces cookies to expire after each session?
I have two boards on the same domain, and users from one can still se the second forum, even though they are not registered, and I tried everything and nothing works... :(

Have you set the cookie path for each board? If you set each one to a different directory they should not interfere with each other.

One of them is on the vary main domain directory, as an .com/index.php and the second is one directory up.

The problem lies in the "remember both dots!!!" .xxx.com and so there is no way to set a separate directory for the .xxx.com as it is, since it thinks it is set for the whole domain...

The problem can be solved easier if you guys made the cookies with an option to have the "per session" expiration...

Have you tried leaving cookie domain blank and putting / for the cookie path on one board and /foldername/ for the cookie path on the other?

Expiring cookies per session would defeat the whole idea behind saving the username and password from session to session.