[bitfolk] Preventing spidering of web applications

Hello, What's the list's preferred techniques for preventing spidering of a web application (in this case Mediawiki) by misguided web robots? robots.txt already in place, but they ignore that of course. Ideally Apache-based. I don't particularly care if they are still able to download the content or not, I just don't want them taking up every single process slot thus impacting non-abusive 'real' web clients. So a rate-limiting solution would be acceptable. Cheers, Andy