Is there a defense against script hackers?

For the last months I seem to find more and more "Page not found" entries in my Drupal logfiles. When looking at these entries I figure that hackers are attempting to inject script into my system. So far they did not get anywhere.

Is there a module that will automatically block the host IP of these script kiddies for an hour if they produce more than three "404 errors" in my logs?

So far I habe been manually blocking these IPs in my access rules which seems like a pointless excercise since by the time I block the IP the attack is long over. And the IP is probably stolen anyway.

Thank you,
Paul