How to block exfinity pop-ups

You will have to explain in more detail what is happening to you since your question is ambiguous.

For example, one thing that happens with xfinity is that it tries to get you to connect to its wifi network. If this is the case, you could try this: go to System Preferences, Network, Unlock if necessary, click on the Advanced... button. Find xfinity in the list of "Preferred Networks" and delete it with the minus button.