I have removed Xfinity many times and it still manages to force my MacBook join even when networks that are top of preference lists are available.
MacBook Pro Retina
In Sys Prefs>Network>Wifi>Advanced>Wifi, after removing Xfinity, have you dragged a good one to the top of the list & Applied?
Require Admin to change?
If you use iCloud to sync your Keychain, then you will be sync'ing all your known WiFi networks, and it is possible that the Xfinity WiFi access point is being repopulated from another Mac, iPhone, iPad or iPod Touch.
Make sure you remove the Xfinity access point from all the systems.
Also as suggested by BDAqua
drag your preferred WiFi network (or networks) to the top of the Networking -> Advanced -> WiFi -> Preferred list.
This is a good idea even with out Xfinity problems, in that the Mac will walk this list in the order arranged looking for networks it can connect to, so if your preferred network is at the top of the list, you will reconnect to WiFi faster.
Why is my MacBook joining Xfinity wifi upon restart when I have removed it from my preferred network list?
