I keep getting pop up screens from Xfinity, how do I block them

AdwareMedic which was developed by one of the contributors of these forums.

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Also check out the following KB Article: http://support.apple.com/en-us/HT6506Remove unwanted adware that displays pop-up ads and graphics on your Mac

Good day njfhotrocks2003,

It is possible to block pop-ups in general but not from specific sites. This article explains how to do it in Safari for Mac OS X 10.10 (Yosemite) -

Safari 8 (Yosemite): Block pop-ups and unnecessary content

Earlier versions you should use this article -

Safari 6/7 (Mavericks): Block pop-ups and unnecessary content

Note that some web sites require pop-ups in order to function normally. If you have issues with a web site be sure to try allowing pop-ups to see if that restores functionality.

Thanks for using Apple Support Communities.

Safe computing,

Brett L