Okk..
If u dn't need it or the data anymore u had to choose the other option..
now all u have to do is to open the VM folder and delete pointer files described Here
Then u have to manually format the LUN itself from the storage software, detach it from the host then represent it again as a new target as a datastore as described in the following KB:
http://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=2004605
The reason it refuses to add it as a new datastore is that, ESXi host still sees it as RDM.
I'm nearly sure that ESXi still records its NAA-ID (Network Addressing Authority ID) as RDM Storage for certian VM.
Wish it helps...