Q: Wifi keeps changing

System Preferences -> Network -> Advanced -> WiFi -> Preferred networks

Remove XfinityWifi from the list.

You can also change

System Preferences -> Network -> WiFi to ask to join new networks, so that if XfinityWiFi is available _AND_ not in your preferred networks list, you will not automatically join it unless you say yes.

Oh yea.  In the Preferred networks list, drag your perferred WiFi access point to the top of the list.  The Mac will walk the Preferred networks list from top to bottom looking to see if it can join any of the networks.  First one that works wins.

So remove ones you do not want to join, and make sure your preferred ones are at the top of the list.

Try setting "Ask to join new networks"

_AND_ under Advanced -> WiFi, delete the xfinity entry from your Preferred list.

Is this in your home?

Do you have a Comcast Cable Modem/WiFi Router?

If yes, then chances are it is Comcasts box that is the source of your pain.  You can call Comcast and tell them to turn off the XfinityWiFi radio in your own home.  Just tell them it is interfering with your home network.

If the XfinityWiFi is from a neighbors equipment, there is very little you can do about that, unless your neighbor is willing to ask Comcast to turn off the XfinityWiFi radio on their equipment.

If the XfinithWiFi is when you are out and about, then there is nothing you can do about that.

But still you should be able to get your Mac to stop connecting if you use one of my above suggestions.