How can I completely disable SW updates on iPhone?
Use Guided Access to restrict their access to the single app during the QA session.

It'll prevent them from accessing anything but that app, but from what it sounds like, that's not a problem, and might even be a boon.

To enable Guided Access:

Open Settings and go to General > Accessibility > Guided Access to set up Guided Access. 



To start a Guided Access session:


Open the app you want to use.
Triple-click the Home button.
Adjust settings for the session, and then tap Start.