Do what dude1234 says and it should eliminate most, but not necessarily all, of the pop unders, but there are some sites where they have figured out a way to get around pop under, or over, blockers.
There is one site I go to several times a week, and no matter what I do they always seem to get a pop under through. As long as web sites make their money through advertising, that's always going to be a problem. You can minimize it, but it's never going to completely go away.