First, make sure you’re not actively using that Comcast network by clicking on the Wi-Fi icon at the top-right of your screen. The network you’re currently joined to will have a check mark beside it.
I have a very clever neighbor.
Then click on the Apple Menu at the upper-left corner of your screen and choose “System Preferences.”
When that application opens, click on the “Network” option, and then if it’s not already selected on the following screen, pick “Wi-Fi” from the left-hand list.
Next, choose “Advanced” at the bottom of the window.
On the drop-down that’ll appear, pick the Wi-Fi tab. You’ll scroll through your “Preferred Networks” list until you find the “xfinitywifi” one; then just select it and click the minus button to remove it.
Confirm that you know what you’re doing on Apple’s scary warning box.
Then you’ll have two final steps: Click “OK” for the current window, and then select “Apply” within the Network preferences pane when your Mac takes you back out there. And you’re done! No longer will you join that Comcast network automatically. Of course, you can re-join it manually if you must, but if you’re not a current Comcast subscriber, it won’t do you any good anyhow.
And hey, what’re you doing going around randomly clicking on open Wi-Fi networks? Be careful with that, guys! Or at least use a VPN if you’re gonna do it anyway—I like Cloak myself.