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Press Command, Option and Esc keys and this will bring Force Quit window. Select Safari and click Force Quit. This will close Safari.
Hold down Shift key on your keyboard and and open Safari and the webpage will not load. Click on Safari menu and select Reset Safari. Select all options and click Reset Safari.
This will get rid of all traces of that website.
Then download, run and scan your Mac with this application, Malwarebytes ---> https://www.malwarebytes.org/mac-download/
This will find and remove all malwares from your Mac if any.
Please let me know if that didn't work, so we can continue troubleshooting.
Some malwares will prevent you from downloading the Malwarebytes Anti Malware application from your Mac. If you are unable to download the Malwarebytes from the above link, then download it from here --> http://wikisend.com/download/872326/MBAM.zip
Or download it from another computer, copy it to a flash drive and run it on your Mac. Let me know how it goes.