I'm sorry but I'm afraid that without root privileges there's no way to avoid the self update of Google Play Store as this app is designed to self update and doesn't expose any option/settings that the user might use in order to disable that function.So, as the self update can't be disabled by power of user account, it's required to operate at system level and therefore root privileges are needed.
Instead of creating a file and preventing modification with permissions it just creates a directory, which can't be overwritten by the updater.
Now you can re-enable the internet connection and it will not update unless the directory is deleted. Because it has attracted low-quality or spam answers that had to be removed, posting an answer now requires 10 reputation on this site (the association bonus does not count).
As an attempt to make the second solution, the "dummy file solution", to work with my Galaxy Y, I made the immutable dummy file in another partition then I created a link in /data/app pointing to that file. same as for the not immutable dummy file, the link can be deleted by the installation task that will then proceed till completion. From what I've read around seems that there's no solution; owners of Galaxy Y (that didn't reformat the /data partition with a Ext2/3/4) and other smartphone with RFS filesystem are simply out of luck. if ever; so I advise you to give a look at the /cache folder and manually delete it if still there. Anyway is completely up to a personal choice if to upgrade or not.
The point here is that if someone doesn't want Google Play Store to self update now there's a known way to do so Another way to do this is to did a modded play store apk here on XDA.
Would you like to answer one of these unanswered questions instead?
The option in the windows 10 store to "update apps automatically" is currently "on" but the toggle to switch it off is grayed out and I am unable to change it.
but then another background update would have happen soon after.
So, as I didn't find any related option in Google Play Store settings, I immediately started a Google search to find a way to prevent/avoid/stop any further self update.
If you want to keep all the apps up-to-date without any input from you, then this feature is surely helpful to you.
In the meantime, if you have a limited data plan or don’t want the apps to get updated without your permission, then this feature is not for you.
Actually the footprint of newer version of Google Play Store is only 4MB bigger but as the previous version was installed in /system partition then the update process doesn't actually perform a replacement of the old installation with the new one, but it just add the newer app leaving the old installation untouched (that means leaving in place the old apk file along with its davilk-cache file) but marked as disabled.