Pop-Up won't go away, now I can't use Safari at all. Help?

I must have typed in the wrong address in a Google Spreadsheet I was working on, because I clicked one of the links and ended up with this:

No matter how many times I click OK it just won't go away. Force-quitting Safari doesn't work because it just re-open my tabs, so this same website keeps re-opening and this pop-up keeps coming back and it won't let me use Safari at all... Does anyone know how to fix this??