It's a JavaScript scam that only affects your web browser, and only temporarily. There are several ways to recover.
1. Some of those scam pages can be dismissed very easily. Press the key combination command-W to close the tab or window. A huge box will pop up. Press the return key and both the box and the page will close. If that doesn't happen, continue.
2. Press and hold command-W. You may hear repeating alert sounds. While holding the keys, click the OK button in the popup. A different popup may appear, which you can cancel out of as usual.
3. From the Safari menu bar, select
Safari ▹ Preferences... ▹ Security
and uncheck the box marked Enable JavaScript. Leave the preferences dialog open.
Close the malicious window or tab.
Re-enable JavaScript and close the preferences dialog.
4. If the Preferences menu item is grayed out, quit Safari. Force quit if necessary. Relaunch it by holding down the shift key and clicking its icon in the Dock. None of the windows and tabs will reopen.
After closing the malicious page, from the menu bar, select
Safari ▹ Preferences... ▹ Privacy ▹ Remove All Website Data
to get rid of any cookies or other data left by the server. Open your Downloads folder and delete anything you don't recognize.
