Should banned user mods/maps/etc be deleted from these boards?

[thejeff]

The warhammer mods certainly could be argued to be derivative works, of both dominions and Warhammer. That doesn't make them public domain. If anything, it makes them copyright infringement. (Assuming them to be done without the permission of whoever owns Warhammers copyright. They can be assumed to have at least implicit permission from Illwinter.)
That still doesn't put them in the public domain. If they're infringing works, they can't be in the public domain, or the holder of the copyright being infringed couldn't do anything to stop distribution.