How can I disconnect a phone from Google's data backup service?
You don't need to do anything. Every device using Google Backup with the same Google account keeps its own separate set of data. When you sign into your Google account for the first time after a factory-reset, the device will look through all the backup sets and try to pick the most recent one from the same device. It'll restore from that set of data, and use that to create a new backup set.

Only if you sign in on a completely new device that you haven't used before (not even before a factory-reset) will it choose a set from a different device: in that case, the most recently backed-up set.

So, you can leave your Galaxy Nexus backing up to its own set, and your Galaxy S5 backing up to its own set. If you factory-reset one of them, it'll restore the apps and data from the same device.

For a much more complete explanation of how Google Backup keeps track of different sets of backed-up data, see my answer to Google Backup: Multiple devices using the same account - what happens on Restore?