Normal users can
chmod files to make them unaccessible like
evgeniy@ubuntu:~$ touch test evgeniy@ubuntu:~$ chmod 444 test evgeniy@ubuntu:~$ echo 'test' > test bash: test: Permission denied
Can something like this be simulated for the root user?
Per Slartibartfast's comments, a few things you should know about chattr and the immutable attribute:
SELinux can be used to mark a file as unwriteable by root in the current domain and user role.