I have escalated your issue to the Comcast Team available to these forums. Check back often for a reply in this thread from a Comcast Official Employee.
HI, @IrisV! I can definitely understand why you wouldn't want them accessing your information any longer. Have you already attempted to delete their profile on your account? It would be logging into the My Account section of the website, users and preferences and deleting their log in information from your account. Let me know if you have or haven't done that yet.