How to block or allow some users to schedule tasks with at or crontab command

Today I was in an odd situation. I was needing to block a user to do crontabs; after a reading, I found the files /etc/{at,cron}.{deny,allow}. By default the .deny files exist and allow does not.

In order 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!