It's common now for ad networks to use "Flash cookies" -- meaning if you clear your cookies, they will just recreate them next time you hit their server using a cookie stored by Flash.

Remove them here: http://www.macromedia.com/support/documentation/en/flashplay...

Be sure to delete both Flash and normal cookies at the same time, since if one is present and the other is not, they will recreate each other. Better yet, use FlashBlock.