Q: Constant ads

Make sure your Mac's OS X and Safari are up to date.

Open System Preferences > App Store then click Check Now.

1. Use  Malwarebytes Anti-Malware for Mac to remove adware.

     https://www.malwarebytes.org/antimalware/mac/

   Download, install , open,  and run it by clicking “Scan for Adware” button   to remove adware.

   Once done, quit Malwarebytes Anti-Malware.

2. Disable Extensions and test.

   Safari > Preferences > Extensions

   Select and disable all extensions and test.

   Enable Extensions one by one and test.

   To uninstall any extension, select it and click the “Uninstall” button.

3. Safari > Preferences > Search > Search engine:

     Select your preferred search engine

4.  Visit the site you want it to be the Home page

     Safari > Preferences > General >  Homepage

     Click the button “Set to Current Page” button.

So, can you please tell us . . .

Are the ads all the same?  Or are there lots are different ads?

Are you seeing ads on this webpage — the one you're using to post your question?

Safari – Popup takes over

Safari – Popup takes over (2)

Popup remover – Scam Zapper

If the pop-up reloads, force quit Safari (command - option/alt - esc) and reopen with the shift key held down. If that doesn't work, disconnect from the Internet, go to Safari/History and delete the website. Quit and reopen with the shift key held down.