A few people have asked about this, so here's a little info.
Spamvertisers are people who flood sites with ads that are often scams. What different about us at craigslist, not unique, but very unusual is that we spend a lot of time every day fighting 'em.
Our first line of defense is our community, flagging them away, and that helps a lot. Some spamvertisers are persistent, and usually people let us know about 'em, then we act. We remove all their ads, block them, and then try reasoning with them. That usually works; if not, I work with their ISP or hosting provider, and that almost always works. When it doesn't, we escalate further.
We've gotten good at this.
I'm only one part of the customer service team, putting in about ten hours per week dealing with such bad guys, the other customer service reps put in more time.
As an example, there's a group that keeps massively spamvertising for what appear to be car and mortgage foreclosure related scams. I finally got a good contact at their ISP and we're shutting 'em down. If they reappear elsewhere, I'll pursue 'em. I've done all this a bunch of times, and will persist.
If you know of people at big sites who are successful at this, please lemme know, getting more allies really helps out.