In the data usage preferences, disable background data. Android allows disabling either for all apps or for individual apps specifically.
Note if you open the app manually, it will still sync as this will count as "foreground".
That option only appears in Android devices that aren't WiFi-only. If you're tethering your WiFi-only device to cellular networks, using a cell-to-WiFi device like Karma Go or MiFi, connecting through a VPN, using paid hotspots that you don't own, etc... the only solution I can see is https://bitcointalksearch.org/topic/m.11863617