This site won't leave me alone. I can't use the internet because it keeps popping up

It's one of those fake scams where it says that your phone is full of viruses but the thing is every time I go on the internet, it doesn't matter what browser I use or what security setting I happen to put on, it keeps popping up and it's driving me nuts. It's even overriding other apps that I'm using and it's driving me crazy. What do I do?