Here is the solution I am testing (Windows):
1. Disable qbupdate.exe using Task Manager > Startup.
Intuit hides the Startup service by naming it Program and disabling the Properties option. We can get around this. In Task Manager > Startup tab, right-click any column header and select Command Line. The new columns reveals the full program path and program name. Now you can identify the Startup entry and disable it. (While you are there, I suggest you also disable Intuit Data Protect if you are not using Intuit's cloud backup service.)
2. Disable qbupdatemonitorservice.exe using Services.
While in Task Manager, click on the Services tab, where you'll see QBUpdateMonitorService, as well as other QuickBooks services. You can stop qbupdatemonitorservice from this window, but it'll start again at next reboot. So instead, right-click QBUpdateMonitorService and select Open Services. Find QBUpdateMonitorService, right-click and select Properties. Under Startup type, select Disabled. Under Service status, click Stop. Finally click Apply and then OK. (Again, I suggest doing the same for QBIDPService if you are not using Intuit Data Protect).
Reboot your computer and I think this should block QB from auto-updating.
