If you need to disable cron for a user completely, you can use the file /etc/cron.deny
To deny a user from using cron, add them to the file /etc/cron.deny using your favorite text editor.
Please note that this does not prevent existing cron jobs from running. If the user already had a cron job before being added to /etc/cron.deny the existing cron jobs would still run.
How to edit a file on command-line
By default, it already contains nobody. If your user is cpaneluser, it will look like this:
# cat /etc/cron.deny
You would need to add all of the users you want to deny from using cron.