How can I completely disable SW updates on iPhone? As a QA engineer I have to maintain a bunch of iPhones to run my tests on them. I have to test on many OS versions, also the very old ones.

The problem is that every once in a while someone(*) (from my or next door team) updates (maybe even accidentally) the OS and we loose a valuable "old OS device".

(*) someone - just an average iPhone user, must be able to manually test apps, not too wide privileges (Do they need access to the Apple ID associated with the phone? No. Do they need to do anything besides access that single app during the QA period? No.); definitely must not be able to update OS

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.