Just because an email is opt-in doesn't mean that users don't / won't click 'report spam' in their email/webmail client etc. without realizing the impact this has.
Sometimes there isn't much that you can do about being blacklisted through mis-understanding of end users. All I can suggest is that you make it easier / clearer for them to stop the emails, and perhaps put some notice in them explaining how they can do it etc.
What made you think that your IPs were blacklisted in the first place? AOL are quite picky and may not accept emails for many reasons (spam is just one), and if you're clean on the other blacklists then...?
Also, with regards to AOL you may want to setup a
feedback loop with them if not already done.