https://www.xfinity.com/support/articles/cancel-my-xfinity-services makes it sounds easy, but don't be surprised if it isn't.
Your best bet is to call them (1-800-Xfinity or 1-800-Comcast) or visit a Service Center or Xfinity Store (check locations and hours at http://www.xfinity.com/support/service-center-locations/). If possible, get written confirmation that your account was closed, as they sometimes "forget". If you have any Comcast equipment to turn in be sure to get a receipt for it and keep that receipt forever.
See http://forums.xfinity.com/t5/Billing/How-to-View-Pay-your-bill-Online-and-or-Secure-your-Account/m-p.... Comcast is in the habit of "forgetting" that accounts were closed and equipment turned in and billing you or turning you over to collection months or years later.
If you need to cancel your xfinity account, can you do it online?
Or do you need to call in. I can't find a way to do it online.
You can't close your account online. To close your account:
Drop off your equipment at any Comcast Service Center or XFINITY Store. Close your account and retain the receipt. Find a Comcast Service Center or XFINITY Store Location near you.
Comcast has teamed up with The UPS Store locations across the country to accept Comcast equipment returns. Simply bring your equipment with you to any The UPS Store location, and an associate will pack and ship your equipment back to Comcast. Visit www.theupsstore.com to find The UPS Store locations near you. The UPS store will give you a receipt with a tracking number so that you can track the progress of your return on UPS.com. Please allow up to two weeks for the return to be processed on your account. Call Comcast 1-800-COMCAST (1-800-266-2278) or 1-800-XFINITY (1-800-934-6489) and close your account.