Cannot unmount or Delete Datastore LUNS

Hi,

I am in a situations where I need some help. I have ESXI 5 running Vsphere 5 with 10 Blades and two clusters, running about 100 + VM's.

I have shutdown/decommissioned some VM's and want to release the SAN space back to be used somewhere else...

However when I try to Unmount or Delete the luns I get following message

Remove datastore XXXXXXXXXXX The vSphere HA agent on host 'XXXXXXXXXXX' failed to quiesce file activity on datastore  '/vmfs/volumes/5010-56cd-f941c-2e5-66db-78e7d1f974-2b'. To proceed with the operation to unmount or remove a datastore, ensure that the datastore is accessible, the host is reachable and its vSphere HA agent is running.

I have tried to couple of suggestions I found on the web, Right Click on the each cluster | Edit Settings | Click on Datastore Heartbeating | then selected Option " Select only from my preferred Datastores, Checked all the Datastores except the ones I am trying to unmount/delete.

Still no luck...

All the help will be greatly appreciated..

Thanks