Apache::Throttle is probably the most elegant solution, if you're mod_perled up - which you will be, if load is such a big deal - and as ever, merlyn has a column on the subject. there's good stuff in the mod_perl cookbook too, iirc.

update: The action taken when the throttle is triggered is up to you, so if you really want to ban someone it would be easy enough to to redirect every subsequent request from that particular IP address to a page that told them off and perhaps offered a chance to appeal the sentence. Or drop the address into hosts.deny, if you really don't like them. Address != user, though...