@leah ouch :(

The following Apache .htaccess fragment seems to have worked well for a phpBB forum I administer:

==
BrowserMatchNoCase "claudebot" bad_bot
Order Deny,Allow
Deny from env=bad_bot
==

(of course it is even better when they don't make it to Apache at all, but I got the impression that they're detecting they're not getting useful responses anymore and then stop trying)