My (sports) board uses different software than this one, but in my case, IP addresses are attached to posts, not users. When I delete a user, all posts by that person aren't removed, but instead just appear as from a 'guest', with the originating IP address still attached. So deleting the user won't help - you have to delete the problematic posts.

And yes, the board owners are legally liable for what's posted by the membership - that's why moderation can get pretty heavy handed when people start crossing lines.