Safari was locked out with the pop-up error code WBACK7917

Safari was locked out and this warning popped up on my screen. "Critical Security Warning! Your Mac is infected with a malicious virus attack.

Please contact tech support at +1-888-307-2735 and provide error code WBACK7917 to scan and resolve any potential threats to your personal and financial information, which was being tracked by suspicious connection." I know it's a scam.

I've had this happen once before and was able to get rid of it thanks to the Apple community blogs. This time, it's not quite working. I've looked it up in the Apple community, yet I still can't get rid of it. I have tried the suggestions of force quit Safari, relaunching Safari holding the shift key, turn off wifi and turn back on. The problem is I can't close that specific tab that the pop is coming through, nor can I open another tab/website whether with wifi connection or without connection. And of course I've tried the simple things first of closing Safari and the computer down. What else can I try/do? Help!!!