Remove Scam pop-up.
1. Click in the menu bar and choose “Force Quit”.
In the window that opens up, select “Safari” and click “Force Quit.
2. Relaunch Safari holding the shift key down.
The pop-up that crashed Safari will not return.
3. Turn off wifi and turn it back on.
Turn off Wifi. Click Wifi icon in the menu bar and select “Turn Wifi off”.
Visit another website.
You won’t have internet connection.
Turn on Wifi. Click Wifi icon in the menu bar and select “Turn Wifi on”.
Select your Network.
4. Delete Cookies
Safari > Preferences > Privacy > Cookies and other website data:
Click the “Details” button.
Remove all cookies except ones from Apple, your internet service provider and banks.
Additional steps,if needed?
http://www.thesafemac.com/tech-support-scam-pop-ups/