Safari Won't Quit

I am unable to quit Safari. I have tried Force Quit in the Apple menu, without success. I have also tried to force quit under Activity Monitor, also without success. Safari simply restarts.

My hunch is that this is the result of my clicking on a bogus prompt to update Adobe Flash, which resulted in (1) pop-under windows for Mac Cleanup Pro to appear on my computer and in (2) Google searches being redirected automatically to Bing.

I'm frustrated.