Here's a way to remove the AOL ads without the need to use external programs.
1. Shut down AIM
2. Open up the AIM program folder, usually located at X:\Program Files\AIM95
3. Delete the file "advert.ocm"
4. Rename the file "admin.ocm" to "advert.ocm"
5. Restart AIM
If you upgrade AIM, make sure to follow the steps above once again.
This may not work with new versions of AOL IM