Wierd, same issue here....Ran Disable Service app and found that SystemUpdateService had been re-enabled again.
The culprit seems to be Google Play Services. Version 4.4 has been rolling out this month... Google automatically pushes out updates to Google Play Services, so rolling back to an older version is only temporary.
Making GooglePlayServices 4.3 a System App seems to prevent it from Automatically updating to v4.4 in the background. (at least for now)
I originally had v2.0.12 as the System App, and tried to just manually install v4.3 apk on top of it, but it automatically updated to 4.4 pretty quickly without prompting shortly after.
Here's what works for now (Root required)
In system settings-apps-Downloaded Apps;
Uninstall Updates to GooglePlayServices, then go to System Apps-GooglePlayServices, Clear Data/Cache, Uncheck Show Notifications, and Force Stop (Don't Disable).
Using RootBrowser;
Delete System/App/GmsCore.apk.
(and GmsCore.odex if exists)
Find 'google_play_services_4.3.23_1069729-036.apk', rename it GmsCore.apk, and replace the original apk in System/App.
Make sure you set the correct permissions to the GmsCore.apk.
Reboot, Set up DisableServices like usual.
I updated and installed a few Apps with PlayStore, and verified that GooglePlayServices is still at v4.3. DisableServices settings have also retained after several reboots.
Phone has been up for a few hours and so far, no OTA Update Notifications
I have not verified the SystemTuner method yet, but I'm assuming with GooglePlayServices v4.3 retained, it 'should' work as well.