To remove Yahoo Search as your default search engine on Mac, you can follow these steps:

1. Open Safari and click on Preferences.
2. Open the Extensions tab, select the Yahoo extension you want to delete, and click the Uninstall button next to it.
3. Open the Search tab, and select a different search engine from the drop-down menu.
4. Open the Privacy tab, and click on Manage Website Data. Select Yahoo and click on Remove.
5. If the issue persists, scan your Mac with a reliable antivirus, such as TotalAV, and remove any viruses from your Mac.