How to protect your cronjob in linux ?

https://en.wikipedia.org/wiki/Chattr

A very useful Linux tip on how to make files and directories undeletable even by the root user. You can apply this tip on all important configuration files so that no one accidentally or intentionally deletes it. To achieve this the chattr (Change Attribute) command is used it Linux. The chattr command “immunizes” the file not only from deletion but also modification. The chattr command does care about chmod values, even if a file has 777 permissions immunizing the file will prevent it from being deleted or modified.

chattr +i /path/to/filename

How to view file attributes ?
lsattr /path/to/directory

How to remove the immutable flag ?
chattr -i /path/to/file

http://websistent.com/make-undeletable-files-and-directories-linux/