Hi Rahul,
You are on the right path. You can unmount it from all the ESXs in the cluster - be sure to unmount it on all hosts that have visibility to it.
If you will UNPRESENT this LUN later, as you have 5.0 I would recomment the following procedure to avoid an APD condition: http://kb.vmware.com/kb/1009449
Hope this helps,
Marcelo