Mac Spyware/Adware Scam!
I inadvertently clicked on an ad in Facebook and the next thing I see is this screen about my Mac having spyware and adware. I've had several clients call me about it but this was the first time I had experienced it first-hand.
It would not let me close the tab nor the window and, essentially, Apple's Safari web browser was locked up and I could not do anything but Force Quit the application. When I relaunched Safari, I got the same window saying I need to call a telephone number for assistance. Based on what my clients have told me, a person with a thick accent will answer and ultimately recommend that you pay them approximately $200 to get this issue resolved. Don't do it!
The fix is simple and FREE: Force Quit Safari and re-launch it with the < SHIFT > key held down. This will prevent Safari from reopening previously open tabs and windows. Then, simply click on Safari in the top-left menu bar and choose, "Clear History and Website Data." A little dialog box will appear and ask you what you want to clear. Usually just clearing today's history, cookies, and website data does the trick. One word of caution: you will lose all open tabs and windows that you had opened before this issue happened so don't be tempted to use Safari's "Reopen All Windows from Last Session" option under "History" in Safari's menu bar or you'll be right back where you started.
As Sgt. Phil Esterhaus used to say, "Let's be careful out there."