I just noticed this as well, and it's breaking an application. As recently as yesterday I was able to uncheck this. Attempts to edit this via the admin URL are also unsuccessful. Didn't see disabling this as part of the new release posted last night. What’s New in ArcGIS Online (July 2015) | ArcGIS Blog
Sync cannot be disabled on a feature service. Once it is enabled and the item is saved the feature service is updated with sync enabled and cannot be disabled. It is forced to enabled because the replicas you create from the feature service will not be able to be synced back if it is disabled and could orphan data. It has always been like this on hosted and ArcGIS Server feature services.
Hope this helps.
That answer assumes that replica's exist. Assuming the replicas have been synced or removed, why can't you disable sync? There are many scenarios where I want to disable sync: I'm making changes to some other app or consumption of the data, or changing the SD, perhaps reposting the data with a new field. I want to turn sync off so some rogue user isn't creating a replica while I'm updating that service. I just recently went through an exercise where I had to query for, and de-register, many replicas on many hosted FS's, and it was a huge PITA. And I'm pretty sure I've been able to disable sync in the past after removing all of the replicas. For example, I can't delete and re-upload a SD if there are replica's out there. Makes sense, as the owner of my own data, I should be able to disable syncing.
After you remove all the replicas you can update the service definition and and set "syncEnabled" : false, at the parent level of the hosted feature service.
There are plans to improve the removal of replicas to have a way to unregister all replicas instead of going through every single one.
Russ, ArcREST/disable_sync_service.py at master · Esri/ArcREST · GitHub don't appear to be working any more, but your solution does. Did the July 2015 AGOL update invalidate ArcREST/samples at master · Esri/ArcREST · GitHub ?
I tested the sample this am and I was able to disable sync. Can you post the error message and we can figure out what is going on. Feel free to log an issue in the ArcRest github repo and we can get it resolved.