1. Force Quit Safari.
Make Safari active.
Click in the menu bar, hold the shift key down and choose “Force Quit Safari’’
Safari will quit.
For another method.https://support.apple.com/en-us/HT201276
2. Launch Safari holding the shift key down.
3. Remove all Website Data
Enable Develop menu, click Develop in the menu bar and then “Empty Caches” twice.
https://support.apple.com/guide/safari/use-the-developer-tools-in-the-develop-menu-sfri20948/mac
4. Remove all Website Data except ones from Apple, iCloud, ISP and your banks.
Quote:
• Remove stored cookies and data: Click Manage Website Data, select one or more websites,
then click Remove or Remove All.
Removing the data may reduce tracking, but may also log you out of websites
or change website behavior.
https://support.apple.com/guide/safari/manage-cookies-and-website-data-sfri11471/mac
5.Clear History.
Click History in the menu bar and then “Show All History”.
Remove all items listed under “Last visited today.
https://support.apple.com/guide/safari/go-back-to-webpages-you-already-visited-ibrw1009/mac
6. Uninstall Adobe Flash Player.
https://helpx.adobe.com/flash-player/kb/uninstall-flash-player-mac-os.html
7. Restart the router.
8. Check for adware.
Run the latest release of Malwarebytes for Mac to remove malware/adware, if installed on your Mac.
For instructions: Install Malwarebytes for Mac v4 Uninstall Malwarebytes for Mac