How do I disable new threads, yet keep replies open?

I have a forum on my site that I use for tutorials, how-to's, etc. I'd like to keep the threads open for replies, but prevent anyone but myself (and ideally, moderators) from creating new threads.

How would I go about this?

If it's not possible through conventional means, is there a way, perhaps a conditional, that would remove the New Post button from that forum ID?

Thanks!