A friend reminded me about chattr. Linux ext{2,3,4} filesystems have supplementary file attributes which can be modified with the “change attributes” utility found in the e2fsprogs package.

The two most interesting in my opinion are +/- i for immutable and +/- j for secure deletion:

$ sudo chattr +i /bin/ps
$ sudo chattr +j secret.txt

$ lsattr bar
----i----j---e- bar