If you are attempting to unmount a NFS command like
# mount -t nfs -o remount /mnt/nfs # umount /mnt/nfs # umount -f /mnt/nfs # umount -l /mnt/nfs # umount -lf /mnt/nfs
Identify which processes tied to the mount need to be killed by using lsof and fuser:
# lsof | grep /mnt/nfs
lsof command above identifies the PID of the processes associated with the /mnt/nfs share. Kill any processes locking the stale mount.
Try to force umount again after the processes as been killed
# umount -lf