How can I stop virus messages in Safari?

1. Effective defenses against malware and other threats - Apple Community
2. Stop unwanted Notifications - Apple Community

No! you do not need or want any AntiVirus software on your Mac. The messages you see are advertisements and the website was paid to pop up that ad. This is usually only the case on websites that are less than legitimate as a website that values their users are more attentive to the user experience over the ad dollars. If you quit Safari with that ad on the screen, it will open to that same page when you launch Safari again. You will need to navigate to another page to get rid of the ad.

For messages that appear in the upper right corner of your Mac, those are Notifications that were allowed from a website. To remove those, you would go to Safari > Settings > Websites > Notifications, and remove all sites there that you have previously allowed to send you notifications. It is not necessary to have Safari open when seeing these type of Notifications.