I use Trillian and it keeps telling me I have AOL Mail. I'm like WTF? I don't even use that. So, when I click on the box, it takes me to AOL but forces me to sign up for some reason to even see the mail. I don't know why I can't sign in with my screen name...
I'm pretty sure there's a way to disable it in trillian as well. It's just buried deep in the options.
Found this on anantech forums, hope it helps:
Go to Preferences, Identities & Connections, click on your AIM account, click Change, go to Miscellaneous tab, uncheck Alert me when new mail arrives for this account