Give them non-editing teacher role? I must admit that I would need to check if that restricts their file system access. The file area in Moodle isn't (by design) spectactularly secure once you have some sort of role in that course. So, if it *really* matters to you it might be better to restrict making backups altogether (to people you trust to delete them again)
Security and privacy
Prevent Teacher Role from Downloading Backup Zip Files
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