As Paggy was saying you do have the ability to do this. If you go to Site Administration, Manage Settings, and check the box: Ads require activation before appearing on the site.
Each Ad that is not activated will not be displayed but obviously is still in the database.
Then you or someone as an Administrator have to go in activate the ad.
What I did was just hooked the site up with paypal, then once they enter in a payment the ad gets activated automatically. I figured most people will not post untasteful material if they have to pay for it
You also have the option to allow users to edit the ad once it is activated. If you dont want them to edit uncheck the box: Allow Users to edit their Ads: