In case of a physical mode RDM you can safely use the delete from disk option, which will only delete the .rdm mapping files, but doesn't touch the data on the LUN itself (i.e. you could re-attach the LUN as an RDM without data loss). For virtual mode RDMs it depends on whether it has active snapshots. These snapshots will be deleted with the delete from disk option, because the snapshots are ordinary .vmdk files.
The LUN will be available for use as a datastore or RDM again, once the mapping file has been deleted. Whether it will work to create a datastore on the LUN, depends on the partition table it currently contains. Depending on this you may need to manually clear the partition table (see http://kb.vmware.com/kb/1008886) before you can format/add the LUN as a datastore.
André