Send a return receipt letter to Comcast Hdq's in Philadelphia saying just what Comcast equipment you returned (e.g. router, etc) and the day you had service turned off. Confirm you do not want to be billed for any of these effective on the appropriate dates. You may also want to have the letter noterized.
I heard terrible stories about Comcast continuing to bill after these date and in fact when I signed off on their rental for a router as I bought my own the next bill had a charge for a rental for the router. Thus another hour call to Comcast.
Save all your receipts for retruns, etc.
