If there were a Recognized List of bad IPs, everyone hereabouts would be very, very happy :)
If a robot is thoughtful enough to identify itself as -bot, -crawler, -spider and so on, you can always block it. There are lots of posted lists of elements that never occur in a human UA. Java, Jakarta, Nutch etc.... Doesn't have to be a complete word. Just match the fragment.
And then un-block things like known google ranges. 66.249, 74.125... (Don't quote me, I'm just making this up off the top of my head and it's too early in the morning.) There's a thread over in SSID called At Home With the Robots that gives a pretty representative sampling of IP ranges for the most active robots.