Today I was in an odd situation. I was needing to block a user to do crontabs; after reading, I found the files /etc/{at,cron}.{deny,allow}. By default, the .deny files exist and the .allow ones do not.
To block a user, I just edit the file .deny and it is done. If you want to allow only authorized users, you need to delete .deny files and create the .allow ones.
Enjoy!
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