Comcast customers can (should imo) opt out of Xfinity hotspot that is enabled on all their routers by default.
From Xfinity Wifi FAQ page:
How do I disable/enable the XFINITY WiFi Home Hotspot feature?We encourage all subscribers to keep this feature enabled as it allows more people to enjoy the benefits of XFINITY WiFi around the neighborhood. You will always have the ability to disable the XFINITY WiFi feature on your Wireless Gateway by calling 1-800-XFINITY. You can also visit My Account at https://customer.comcast.com/, click on “Users & Preferences”, and then select “Manage XFINITY WiFi.”
Also, in most cases, you'll want to use your own wireless router. You'll need to call Comcast support and have them put their router in bridge mode. With the their router in bridge mode, you can simply plug a network cable from the Comcast router (they say port #1) to the WAN/INTERNET port on your own wireless router and have fun!