acknowledged your cancellation - they must have done, as you say they agreed to send you a refund. That they haven't done so, and that they are still charging your credit card account is clear indication of an administrative eror - one that needs to be corrected immediately.
I assume that you have a copy of your original cancellation email, and the one from AOL promising the refund?
What you should now do is:
1. Write a letter to the debt collector, stating the facts, and telling them that they should not pursue you further in this matter. This is important, as without any documentation you may find yourself confronted with a county court summons for defaulting on a direct debit agreement.
2. Write to AOL by special delivery, again stating the dates and the facts. Tell AOL that they must stop the debt collection agency from harassing you immediately.
3. Write to Capitol One giving them the facts, and enclose a copy of the original email you sent to AOL, cancelling the agreement.
It's important that you act very quickly, and that you draw a line in the sand by sending all three letters - do it by post, and not by email. As a general rule it's vitally important that any cancellation of a direct debit agreement is done in writing, by post, and that a record is kept of any subsequent phone calls. Once a debt collection agency becomes involved you are dangerously close to a court action, and your credit rating is at risk.
Please post back to this thread and update us on the outcome of this would you? And send those letters today if possible.