If you are still on the old email format-------------------
The auto-complete feature in the Comcast address book has it's own memory. You can't access it or alter it. Periodically Comcast will re-set accounts, and at that time entries that have been deleted from your address book should vanish. However, they only do that sporadically, on no particular schedule. So those old addresses could keep popping up for quite a while.
Also, when you type an address into the To: Cc: or Bcc: fields, the feature will remember it if you actually send an email to that address, even if it’s not in the address book. It's actually pretty common behavior with webmail accounts.
If you are in the new format, -----------------you have more control. From the Inbox page click on the gear icon in the upper right corner of the screen, then on Settings>>>>>Mail>>>>Mail Settings. Under Common you will see the option to enable or disable retention of email addresses in the Collected Addresses folder. That folder is found in the address book, and there you can delete contacts that appear in the auto-fill.