Your system is blocked by spammers. If you see any number on the screen or in that message, then don't call that number. Follow the steps below to fix it.
Press Command, Option and Esc keys and this will bring Force Quit window. Select Safari and click Force Quit. This will close Safari. If it is not closing, then hold down the Power button of your Mac for 5-10 seconds and this will turn it off. Then turn it back on after 2 minutes.
Then hold down Shift key on your keyboard and open Safari. Click on Safari menu at top left next to Apple menu and select Clear History and Website Data. On the drop down next to Clear, select All History and click Clear History.
(If you see Reset Safari instead, click it and Reset Safari)
That will remove it. If you are using different browser like Google Chrome or Firefox, then let me know.
If you have any difficulty on following the steps above or if you are unable to remove it, please reply in here and I will continue to help you. Please let me know how it goes.