The forum is just a platform for posting information. Such as, for example, LiveJournal, Twitter, Vkontakte.
If the administrator is not acquired article user, respectively, it does not have the rights to it and can be free to quote/use. There are also examples, when the administration on the forum pays for the material.In this case, the copyright may belong to the administration of the forum.
Also, from a copyright law need the following:
\r"Are not objects of copyright:
1. official documents (laws, judgements, other texts of legislative, administrative and judicial character) and official translations thereof;
2. state symbols and signs (flags, emblems, orders, banknotes and other state symbols and signs);
3. works of folk art;
\r4. reports on events and facts that have informational character."
Forum posts fall under 4-point, so to speak, the public.
So if the theme itself has no marks, indications of the type "written at the request of the administration, all rights belong to the administration", feel free to take material.
Besides, do not forget that the text still belongs to the author and he can also post it anywhere without asking the admins of the forum.