I need to disable 'Sync' on a Hosted Feature Service that contains 2 layers BUT the check box is grayed out.
I find that I CAN disable/enable Sync on other services I have authored that only contain 1 layer.
I usually temporarily disable sync when I'm overwriting a service as replicas are removed, allowing subsequent overwrite. The other method of replica removal (from the REST end point URL - un-registering replicas one by one) is tedious as this service is widely consumed for offline Collector data creation an many replicas, created.
See screenshot below for the 2 layer feature service with the grayed out 'Sync' control: