Hello ​@whoistim 

Thanks for your post. Some users prefer to authenticate on Strava using their Google account login credentials. Sorry, there is no way to disable this prompt or prevent it from showing on the login screen.

I find it useful, now more than ever.  It’s an easy workaround to bypass an email with that new 6-digit sign in code. 

I get it. Google provides an API to unify / simplify logins. And that has value for some users. 
The sucky thing is there is no way for the end user to disable the prompt. That’s just how Google implemented it. And Google has zero incentive to do otherwise. So the rest of us who might never use the feature will have to see the prompt and dismiss it every time we use Strava (or any site which uses the feature). Forever. It’s a perpetual ad for Google.  
Oh well. Google provides a lot of stuff for free. The trade off is that they can be insensitive to end users. Whatever. 

​@whoistim I was able to confirm that the feature you are describing is provided by Google. If you are using Google Chrome, you can read about disabling the pop up on the Google Chrome Help Center: Learn about third-party sign-in