I've seen this in the past with NAS Datastore sometimes and you can also execute the destroy command from Service Console using vimsh, so long as you've migrated everything off and it's okay to destroy the datastore, try the following:
vmware-vim-cmd hostsvc/datastore/destroy <datastore_name>
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William Lam
VMware vExpert 2009
VMware ESX/ESXi scripts and resources at:
VMware Code Central - Scripts/Sample code for Developers and Administrators
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