As you state you don't even need "that app", here are two approaches to keep it from eating your battery:
Disabling the app
- Go to Settings → Apps, select the "All" tab
- Scroll to the app in question (in the current example, Google+)
- Tapp its entry to open the details
- Tapp the "Force Close" button to end all its processes
- Optionally, tapp the "Clear Cache" and "Delete Data" buttons to free some space on the device
- If you see a button labeled "Uninstall Updates"1, tapp it. The button will then rename itself to...
- Tapp the "Disable"1 button2
The app should no longer start itself now, and thus not eat any battery anymore. If you need it back at a later point in time, it's still listed in Settings→Apps (though it might have been moved to the very end of the list). Opening its entry should show a button labeled "Enable" now, which speaks for itself.
Keep the app from running when you don't need it
On the playstore, you will find an app named Greenify. This can be used to automatically "hibernate" an app when its not running in foreground. Though Greenify is delivering its full potential benefits only with "root powers", it can do many things already without. If its free version cannot deal with the app in question (Google+ here), you might need the Donation package (for ~USD 3), which has additional ways to force apps into "hibernation".
Greenify: Start screen, App Analyzer (Source: Google Play; click images for larger variants)
1 If you have none of the two buttons, but an "Uninstall" button, it's no system app: simply uninstall it.
2 Note that not all system apps permit this (button is grayed out then).