[kh99]

It looks like forum passwords are saved in cookies with long expiration times, and the cookie name is COOKIE_PREFIX . 'forumpwd'. So you might be able to make a plugin that deletes that value from $_COOKIES early on, like in global_start or something. But I think that might result in a user having to enter a password every time they click on anything in a password-protected forum, which might be too much.

I think if you had the cookie saved with an expiration time of 0 then it would only last until the browser was closed, which sounds more like what you want. In forumdisplay.php around line 140 there's this code:

// set a temp cookie for guests
if (!$vbulletin->userinfo['userid'])
{
    set_bbarray_cookie('forumpwd', $foruminfo['forumid'], md5($vbulletin->userinfo['userid'] . $vbulletin->GPC['newforumpwd']));
}
else
{
    set_bbarray_cookie('forumpwd', $foruminfo['forumid'], md5($vbulletin->userinfo['userid'] . $vbulletin->GPC['newforumpwd']), 1);
}

and that '1' as the last parameter in the else means that the cookie will not be temporary. So you could edit that file and take out the 1 (or really just comment out everything except the first call to set_bbarray_cookie()) and that might do what you want. (There may or may not be some way to get the same result though plugins, I don't know).

BTW, I haven't actually tried any of this.