Like Gmail and Google Maps, Google+ comes preinstalled on phones running Android 4.0 (Ice Cream Sandwich) and above. While you can't uninstall these applications, you can disable them to prevent them from running. To do this, go to Settings > Apps > All > Google+ > Disable. Note: You may have to select Uninstall updates if you downloaded a newer version of the app from Google Play. Learn more about optimizing your memory usage on your Galaxy Nexus.
On earlier versions of Google+ (prior to version 2.6) on Android 4.0 and above, data used to sync your photos from Picasa to the Gallery app would be attributed to Google+. This is because Google+ and Gallery shared the same sync setting ("Sync Google Photos") in versions prior to 2.6. If you had any photos stored in Picasa when you synced your photos down to the Gallery app, then this is most likely why you're seeing that data attributed to Google+. You can restrict the Gallery app to Wi-Fi only by selecting Sync on Wi-Fi only in the Gallery app Settings.