What output do you get from lsof 'mount point'
Is there anything there that can be killed ?
As to the processes that cannot be killed, do a ps -el | grep 'suspect process name'
If the PRI field is very low then the process is so far asleep then it is not even looking for the kill signal, this is often the case with a process that is waiting for some resource or other.
The only way I know round that other then a reboot is to make whatever it's waiting for available (i.e if it's looking for a nfs volume, make sure it's available)
Wayne