You'd be shocked how much this happens. I'm pretty sure there's some kind of recourse against it... try emailing the Craigslist people about it. Or you could just flag the ad.

I remember a few years ago putting a "big movie collection" up in the free section, with a picture in the description of a box full of porn movies, and a buddy's phone number on it. People calling him a few times an hour for his Big Box of Porn.... that was good entertainment Took it down after a couple days though.