There are a number of popular scams that involve displaying a pop-up window that prevents you from doing anything with your browser. These scams usually involve telling you that you have a virus and that you need to call a phone number to get rid of it, or that you owe some law enforcement agency a fine and must pay it to unlock your browser, or some similar nonsense. This problem is not usually caused by adware, just a JavaScript on a scam web page.

To get rid of the pop-up, as well as learn more, including what to do if you actually called the number given in the
pop-up, see:

http://www.thesafemac.com/tech-support-scam-pop-ups