First, use lsof to see if there is anything using that share, then take the appropriate action.
For example, I had a box that was disconnected, so my load average was messed up. Here's what I did..
lsof | grep boxname | less
This gave me a line that said something like:
bash 19370 myname mem REG 3,1 8330 394003 /mnt/boxname
Then, opening top, typing k to bring up the kill interface, typing in 19370 (the PID) and then selecting a kill signal of 9 solved my problem. I was then able to
since nothing was requesting it.
Yes, you could kill it right from the command line, but it's more satisfying to watch it disappear.... :-)