How to prevent site being hammered?

Depending on your forum software you can look to integrate one of the stopforumspam.com plugins. This ties back to their DB & with the various plugins can give you a decent amount of control over bots and other nefarious IP's that might be trying to hit your system.

If you have just a few IP's that are the worst offenders you can always use CSF to block them. Another thing you could use is CloudFlare's free protection. It offers an option to state that your under attack and uses a challenge page to determine if the person is real or a bot.

 

You need to figure out what they're targeting. If it's trying to make posts/comments then generally they can be deterred with a captcha or something similar. Once they realize they're not accomplishing anything they'll stop.

If it's a DDoS-style bot traffic aimed at taking things offline or simply scraping content...well that's a lot harder to deal with and yeah if they've reached you it's too late to really do much about it. You need to stop those before they get to you with something like Cloudflare.