Hrm I think this could be achieved with permissions - If not 100% at least the no Mods/Admins part...
I will test if it's possible without modification but the modification you would need to make would be simple enough
I should warn you though that the information and attachments are never going to be 100% unseen - Your host can access the data via the database interface (usually PhpMyAdmin) and if your host has strict rules concerning adult content or pornographic content then it is possible that your users may inadvertantly drop you in it...
It may be a good idea to allow Administrators the ability to view threads and attachments in these "private" forums to ensure for your own sake that nothing un-toward is occuring
Chris