If youre that large, you should really think about using a Dedicated Server or VPS; its completely understandable that you utilize those resources with a large forum.
If youre getting that much TRAFFIC theres not much you can do besides move to a VPS or a Dedicated Server (if funding permits I recommend the dedicated option).
If it was simply boggy scripts id say to police your webspace, but with that kind of traffic it's likely just standard browsing.