I looked into a copyright provision about compilations as suggested in one of the threads in this forum (Forum owners own the collection of the posts rather than posts themselves), however, forums do not qualify as a copyrightable compilation:

Here is the definition (all three elements must be present for the compilation to be considered copyrightable):
1. the collection and assembly of pre-existing material, facts, or data; (forum posts are not pre-existing material)
2. the selection, coordination, or arrangement of those materials; and
3. the creation, by virtue of the particular selection, coordination, or arrangement of an original work of authorship.

So, how do I protect my forum?