Sorry, gang, you're wrong on this one - copyright belongs to the author of the post, unless s/he specifically & explicitely gives it to the publisher.
This is one of the few bits of internet copyright law that's pretty clear: some forums state, in their terms of use, that all posts become property of the forum - and have lost court cases over them.
The copyright's still yours: they can't (for example) sue you for publishing the same post elsewhere. And they can't (another example) publish a book made up of all your posts (unless their ToU specified you to be granting them a full licence, like some websites do).
What you DO grant them, by posting, is an implied (limited) license to display your posts.
So in practical terms, in this case, the upshot's the same: you granted them the right to publish, and there's no way you can force them to delete your old posts. It'd be like an author demanding that his publisher track down & burn every copy of a book he wrote...