Thanks for reply.
I know it seems there are easily accessible solutions out there.
But I found out that it doesn’t really “hide” or “secure” my forum,
rather lead out-siders, who’s not really welcome to my forum,
to register.
(I am planning on to modify register and log-in system in the bbpress somehow,
so that only the accepted people can join the forum)
I know this is quite frustrating problem,
since this is not frequent asking or ideas about forum.
(Who ever would want forum to be completely hidden?)
Regard.
Ariel.