There are a few ways you can do this, but not without a few downsides.
You can’t block OS updates altogether, but you can lock down the device in Kiosk mode so as to not let the user confirm Install to proceed with the OS updates. You can add the essential apps in the Kiosk but the notifications bar should be kept disabled to disallow update confirmation.
First, the devices need to be supervised. You can use Apple Configurator or DEP to supervise.
You can turn on Single app Kiosk mode but then the device needs to be locked down to a single app the whole time.
If you configure a proxy server on your Network and have the iOS Update servers blacklisted, then using Hexnode MDM, you can deploy a Global HTTP Proxy configuration on the iPads to force all traffic through your Proxy server. Blacklisting iOS update servers ensures that the updates stay unavailable on the devices.
If you don’t mind doing this the non-MDM way, then you might want to take a look at this blog.
Thanks and regards
Hexnode Support Team
+1-866-498-9407 (US Toll Free)
+44-(800)-3689920(UK Toll Free)
+61-1800165939 (Australia Toll Free)
Hexnode | Mitsogo Inc.