How To Block Someone in Google Photos Via iPhone?
And then, only those you share with see them.
So, that is not true. Google Photos are basically secure, but once you share a photo with somebody (anybody) and they get the URL, there is nothing to prevent them (or anyone else) from seeing the photo from that URL. That URL is not account protected or anything like that. The URL is pseudo-random and unguessable, but once it is out there and shared with anybody, anyone who connects to that URL will see the photo.
See https://www.alexkras.com/do-not-share-your-google-photos/ and https://www.theverge.com/2015/6/23/8830977/google-photos-security-public-url-privacy-protected
So, to stop sharing any photo, go to https://photos.google.com/sharing on a computer browser, and it will list all of the photos that you have shared. You can turn off sharing for any photo that you like. Do that and nobody else will be able to see the photo.
If you put it that way, then regardless of how you share a photo, anyone can see it if the person you share with shows it to others. But the OP didn't say they had shared a url to anyone. They only asked how to block someone. More info would have helped.
Yes, you are right. But I just wanted to correct the part of your post that I quoted. Google Photos has no way to share a photo with only specific users by permission. If OP has shared a photo to more than one person, and wants to stop sharing to only one of them, you cannot turn off the share to that one person. I believe you'd have to stop sharing that photo completely, and then share again (which I believe would create a new, obfuscated URL) to the smaller group of people.