The ads help generate a small amount of revenue to help offset the costs of running the site which are fairly substantial. Your paid membership also helps with those costs but there is no way to remove the ads on a per user basis.
We're working on reducing the ads back to like it used to be, but like Spencer said, they will always be there in some format. We don't have enough paying members (since it's voluntary) to cover all the costs. But, your contribution DOES help- there would be way more ads if people did not contribute.