WordPress MU

Since I set up my blogs I have been inundated with spam. To combat this, I would like to require that people complete a human authentication process in order to comment on the blog. I see this sort of thing all over the internet where a blurry image is presented and we have to read it and type in what it says. Is there a wpmu plugin that does this?

Thanks

Please do a search for captcha here in the forums. It's been discussed at some length. (I believe some of the plugins require some edits to get them to work on mu. That's why I'm suggesting the search.)

Tried this ?
http://wpmu.org/wp-hashcash-for-wpmu/