In config-dist.php of version 3.4 ...
// Completely disable user creation when restoring a course, bypassing any
// permissions granted via roles and capabilities. Enabling this setting
// results in the restore process stopping when a user attempts to restore a
// course requiring users to be created.
// $CFG->disableusercreationonrestore = true;
Have not tried it on any site. Wonder if this kicks in after having stepped through the beginning of the restore process where person restoring has option to de-select users.
OR if the setting looks for an extracted users.xml in moodledata/temp/backup/[hasheddir]/ and if not blank, then stops the process.
So some 'education' of those that do have ability to restore, might be in order to avoid their frustration, but if they have been informed it will happen and they have to attempt restore again .... well, what can I say ... If it hurts don't do it!
Bout the only way I can think of to avoid any mistake by person restoring, etc. is to edit the xml files in an archive of the backup, removing users, then re-archiving so the restore can proceed without restore user having to think/do anything.
While the above could be automated via bash shell scripts, if one has tons of courses, could take some time. And, all that could be circumvented if person restoring has a backup of their own in possession .... wouldn't be accessing the file system repo containing the 'scrubbed' .mbz backups.
Since you are in trials, add the line to config and try one restore with users.
Depending upon how it behaves you might have to monitor moodledata/temp/backup/ for a while to see it's being cleaned up as it should via cron/task.
'spirit of sharing', Ken