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!