How to block Optimum pop up
I have a Mac Mini (Late 2014), processor 2.6 GHz running OSX 10.11.1 El Capitan. My problem is a popup that keeps coming up constantly, no matter what program I'm running. The popup ask me to sign on to Optimum WiFi but I don't have and never had Optimum. I had done all steps recommended by Apple support, deleting a lot of files. I scanned with Sophos Anti virus, Clean my Mac and Sweper, all report my Mac “clean”. I have blocked pop ups both in Safari and Chrome. Short of a new clean install does anybody had the same experience and can tell me how to get rid of this nuisance? This thing has come up twice while I right. Thanks a lot.
Mac mini (Late 2014), iOS 9.1