This may not be 100% on topic for linux, since contemporary versions
have forced and lazy umounts, but I came up with a way of making it
easier to umount a filesystem on an AIX box, when the NFS server was
gone forever, and we didn't want to reboot the AIX box.
The trick was to create a virtual interface with the same IP address as
the server that was gone forever. It then became far easier to umount
-f the filesystem. :)
Only thing is, the DNS resolution on the AIX box got semi-screwy due to
that virtual interface somehow, so it was important to destroy the
virtual interface after the umount had succeeded.
HTH's someone.