How to disable Google One Tap sign-up prompts?

You can disable these prompts from your Google account's permissions page:

https://myaccount.google.com/permissions

Or go into your Google Account Settings and to the Security Tab. Here you will find a section "Signing in to other sites". Select the "Signing in with Google" link to get to your permissions site.

Google Account sign-in prompts

Allow Google to offer a faster way to sign in with your Google Account on supported third-party sites

Toggle off "Google Account sign-in prompts" to disable these prompts across the web.

Don't be fooled by the toggle since it currently doesn't seem to display the toggle state correctly. Click it once and refresh.

In your Adblocker, block all content from smartlock.google.com. That will be the end of these popups.

This popup is provided by a Google service website owners can select to integrate on their websites.

Daniel's answer works on medium.com^* (and probably others) but not in pinterest. That site keeps nagging you on every visit, and uses pop-up elements containing multiple 'random' ID strings (so you can't just filter on these). The substring to filter on for blocking the popup with AdBlock Plus seems to be:

&origin=https%3A%2F%2Fnl.pinterest.com

^{* Note that medium.com prompts you only once anyway; your (non-)response is kept in a cookie.}