Is it possible to block macOS Mojave installation?
You can probably use a MDM solution to support all your devices (mobiles &amp; desktop eg: Miradore, Jamf etc.)

Check this support document:


Mobile device management settings for IT - macOS restrictions.


You can temporarily block any update (to secure for example a shared mac). Use 

sudo spctl --disable --label "Mac App Store"

to block any installation from the Mac App Store.

You should create a standard and an admin user and give the standard user credentials.

Using imaging or Net boot is not the recommended approach since the introduction of APFS in macOS High Sierra 10.13.