Dynamically blocking excessive HTTP bandwidth use?

If your PIX is running version 7.2 or greater of the OS, or can be upgraded to it, then you can implement QOS policies at the firewall level. In particular this allows you to shape traffic and should allow you to limit the bandwidth used by bots. Cisco have a good gudie to this here.

I'm not sure about yahoo, but you can configure the frequency Google's bot indexes your site. Have a look at Google Webmasters. I'm not sure if Yahoo has anything similar. At any that'll reduce your traffic up to 50%.

Alternatively, some web servers can limit traffic per connection so you can try that. I personally would stay away from hardware solutions since it's most likely going to cost more.