If you need to make your backups undeletable, even by root, the chattr command is the solution.
Chattr changes the file attributes. Chattr +i /path/to/file sets an invisible lock on the file, and makes it unchangeable, so undeletable. The +i bit is called immutable bit.
Make the file unchangeable:
$ sudo chattr +i /path/to/file
When you do an ls -l on the file, you will not notice anything awkward, the +i bit is invisible. To see the hidden bits, use lsattr /path/to/file
$ lsattr /path/to/file
----i-------- file
So, an file with an immutable file cannot be deleted, even by root. To delete a file with the +i attribute, remove the +i bit and than do the rm.
This is how you remove the +i bit:
# chattr -i /path/to/file
Now, with the +i bit removed, everybody having write permissions can remove the /path/to/file.
The immutable bit, proof of power – with examples:
# chmod 777 file1
# chattr +i file1
# rm -rf file1
bash: file1: Permission denied
# echo "test" > file1
bash: file1: Permission denied
# mv file1 file2
bash: file1: Permission denied
# chattr -i file1
# mv file1 file2
# ls
file2