If you use the latest version of AIM (//www.aim.com) (v5.5.3572) and you are using Windows, you can download a patch to get rid of this nuisance. I created a patch that you can download that will get rid of the ads completely.
To download go to my website, ModTweaks.com (//www.modtweaks.com) and download the version that is right for you. You can also view screenshots and learn exactly what the patch entails at the site.
Enjoy!