I don't really have a fool proof way yet. I do approve all the job postings before they go out. I think it might be hard to profile job posters because a lot of them are probably going to be first time posters.
I totally understand as you would need to somehow attract job postings to bring more developers to the site (or vice versa). I guess once the site picks up, and both companies and freelancers see the value, then you can come up with a way to verify jobs. Best of luck.