Honestly not sure of a reasonable use case for disabling the back button as it is a universal Android paradigm and is therefore pervasive throughout the OS. iOS has swipe from the side or integrated back buttons within applications but Android and its apps are very much designed around the fact that there is an accessible back button. 

With that said, if you still think this is necessary, you can re-program any of keys within the Zebra KeyMappingMgr CSP which can be distributed and applied as MX XML from SOTI.

I suggest you read up on Zebra's techdocs if you are not familiar with KeyMapping as there are 100s of other settings within MX. 

https://techdocs.zebra.com/