"it's your neighbors"
I suspect "xfinitywifi" works in the same way as other similar services that provide a service to allow other Xfinity users to gain Internet access when they within range of such a hotspot. My router does a similar trick to allow other customers of my ISP to access the Internet, but not my private network, when they are away from home and within range of my ISP WiFi hotspot, or any other of over a million similar hotspots around the country. If I turn off the WiFi int he router, it disables the WiFi for my private network AND is disables the ISP hotspot as well. They both use the same transmitter circuits in the router. Another setting that is common is the specific wireless channel that I have configured the router to use. The ISP hotspot simple adds the ISP-WiFi SSID as a second SSID; however, it uses a totally separate logical network to connect to the Internet. My devices on my private network get addresses starting 192.168..., while the ISP hotspot SSID assigns addresses starting 10.... The two networks are totally separate. If I opt out of providing the ISP hotspot service, I can no longer access any of the hotspots around the country. However, it can take several days before a change in the account setting switches the hotspot on or off in the router. Sorry, this is a bit of a long answer, but I hope it helps. So - the XfinityWiFi SSID will probably go on or off but it might take several days for a change to take effect.
No. If they have they 'broadcast SSID 'disabled on their router you will not be able to see their network.Its best to change the name of the the network because I believe some devices still allow you to connect if you know the network name and PIN and manually type them in.Whenever you change the settings on the router you need to hit the save button and restart the router to be sure the changes you made are implemented.