How do I remove virus Startpage Search,com from Vivaldi browser?

I spent the past 3 days working with a 2011 Imac with Sierra 10.12.6 and a 2015 Imac with Sierra 10;12.6 . After deleting the non-admin user from the 2011 I did a full clone migration into the 2015 Imac. Next I set up new non-admin user without downloading any thing from the internet. While I was working with it a popup interrupted shutdowm to tell me that another user was using the computer. Today I turned it on again and went online using Vivaldi browser. While i was studying articles about exporting bookmarks and paswords. The site pages went crazy then a new but identical but full screen page apeared with a large logo in the upper left corner that said Start Page Search. I could not control the computer so I did a hard kill with power switch, waited 5 minures and started it again. I quickly opened Vivaldi Preferences > Search and found among the search engines the name Startpagesearch.com. It would not delete and it made itself the default search engine. Bitdefender does not see it even though I have unprotected all files so it can search everything. I quickly opened Vivaldi Preferences > Search and found among the search engines the name Startpagesearch.com. A web study revels that this is a tool that employers use and it can be configured to be unseen by antivirus. On the web there is considerable discussion about how to delete with Norton but here at Bitdefende it seems that none of the discussion participants have ever heard of it. Please help. I cannot use my new computer.