Join Xfinity Comcast ad pop up

Ever since the Apple store in Cherry Hill, New Jersey reloaded my software there is a pop up advertisement I cannot for the life of me block, turn off or get rid of. It is a Comcast pop up advertisement the Join XfinityWifi one. No matter what I do it won't stop and is getting worse! It opens whenever I turn on the safari. I do not nor have I ever or will ever use Comcast Xfinity wifi. I have had for many years and always have used Verizon Fios. How can I turn off or block this annoying ad?

thank you,

Steve M Pitman,NJ