There are two ways..
you can use aimazing that just overwrites well actually superimposes an oscilloscope on the ads.. or
let me go find it on /. yah I knoiw.. but I found it there
Here's how you disable AIM Advertisements:
Make sure AIM isn't running (not even the icon in the tray). Use notepad to open aim.odl. Scroll down to where there are two sections that have "advert" followed by "required". Remove the word required in both cases, and save the file. Now delete or rename your advert.ocm file (I renamed mine advert.bak). Now start AIM. When you sign on, no more annoying advertisements at the top of your buddy list.
Enjoy.
Gatsby