How to block apps on tablet from accessing the Internet while on tethering I use my WiFi-only tablet for turn-by-turn navigation.  I either use wifi hotspot tethering or bluetooth tethering through my phone's data plan.  (Or USB tethering once I get a Y charging cable.)

My problem is that my tablet apps run their background updates and syncs while I am tethered, consuming my limited data plan.  The worst culprit is Google account autosync.

While most apps nowadays are wifi/3G-friendly, that is, they have a setting to do heavy downloads only on WiFi, they do not differentiate broadband wifi or my phone's wifi hotspot.  The Play Store app on my tablet would particularly update large apps in the background while I am driving!
In Android 4.2 and later, you can tell Android which Wi-Fi networks are mobile hotspots. In the device Settings, go to the Data usage page, then open the overflow menu (the three dots in the top-right) and click Mobile hotspots. It shows a list of Wi-Fi networks known to the device: tick any that are mobile hotspots.

Then, when you're on a network that's ticked on that list, Android will tell apps that the device is on 3G. Any "Wi-Fi only" update settings in apps or in the OS will act accordingly.

In addition, if the mobile hotspot is an Android phone running 4.3 or later, the hotspot will communicate this to the tablet, so it will automatically treat that Wi-Fi network as a mobile hotspot, without any user intervention.

For earlier Android versions, certain apps (such as my app Showr) have similar settings to the Android 4.2 mobile hotspots page, allowing you to treat a Wi-Fi network as 3G within that app. This is less useful than the Android-wide setting, because not every app supports it, and you need to set it for each app individually, but it works just as well if you can't update to 4.2.