Asked, answered, and more or less beaten to death a while ago: Google Popup Signing Prompt - How stop it? - Apple Community
Short answer: You can't. "Sign in with Google" is a website API that only the website can choose to include or exclude, just like any other website element.
Answers are difficult to find even on this site, for reasons similar to what you described. Most of the proposed solutions tell you to go into this or that Google setting. If you have no Google Account, there is no setting for you to alter.
Workaround: Use Distraction Control in Safari to hide items on a webpage - Apple Support. That will not make the intrusiveness disappear, but it will be rendered less annoying.
Otherwise, boycott the website and tell them why. They won't be interested, because websites cost money, and Google gives it to them in exchange for that API and the advertisements they host.
Search "how to block sign in with google" - Apple Community if you so choose, but that's the answer.