As you are in Apple Help I assume you are running a mac of some sort?
If so switch it off then disconnect it from the internet. Start it up, still disconnected, and open up your browser. Clear all of the history from your browser and reset it then switch off your mac.
Reconnect to the internet, switch on and run your browser and all she be ok.
If you are running a PC then Google mutebutton.info how to remove pop up.