macOS Safari 11
1. Use MalwareBytes for Mac to remove malware/adware.
Install guide: https://support.malwarebytes.com/docs/DOC-1817
Uninstall Guide: https://support.malwarebytes.com/docs/DOC-1190
The installer may ask you to allow it in Security & Privacy /System Preference. Allow it.
2. Disable all extensions and test.
Turnoff or uninstall extensions.
Section: Manage extensions: https://support.apple.com/guide/safari/use-safari-extensions-sfri32508/mac
3. Choose a search engine.
Delete all text in the Smart Search field, click the magnifying glass , then choose a search engine from the list.
https://support.apple.com/guide/safari/customize-your-search-ibrwe75c2a3c/mac
4. Set your Home Page. https://support.apple.com/guide/safari/set-your-homepage-ibrw1020/mac
5. Restart your Mac.
6. Launch Safari holding the Shift key down.
iOS 11
1. Force Close Safari. https://support.apple.com/en-us/HT201330
2. Clear history and website data from Safari: https://support.apple.com/en-us/HT201265
3. Stop pop-up ads on Safari
'Pop-ups and ads in Safari
If you see an annoying pop-up in Safari, you can go to the Search field and enter a new URL or search term to browse to a new site.
If you don’t see the Search field on your iPhone, iPad, or iPod touch, tap the top of the screen to make it appear.
Some popups and ads have fake buttons that resemble the close button, so use caution if you try to close
a pop-up or ad. If you're unsure, avoid interacting with the popup or ad and close the Safari window or tab.'
Section: Close pop-up ads on your iPhone, iPad, or iPod touch https://support.apple.com/en-us/HT203987
4. Avoid scams: https://support.apple.com/en-us/HT204759
5. Identify legitimate emails from the App Store or iTunes Store
https://support.apple.com/en-us/HT201679
