Under Linux I can:
chattr +i SOMEFILE
so that even root can't modify the SOMEFILE. Are there any similar solutions for AIX?
At first I thought that ACLs could do the job. But a quick google revealed that the owner and root can (re)set ACLs on AIX.
A workaround might be using an NFSv4-mount from another server. In that environment root normally is mapped to a different (not privileged) UID.
The wikipedia page on chattr says: chattr is a command in the Linux operating system that allows a user to set certain attributes on a file residing on many Linux filesystems. chflags is the analogous command on modern BSD systems, including OS X. The commands are similar to the attrib command on DOS, OS/2 and Microsoft Windows. Other Unix systems have no analogous commands; Solaris supports extended file attributes, but there are no user level commands for setting or changing them, and the chatr command in HP-UX, and chattr in AIX have unrelated functions.