You can delete the datastore or unmount it then detach the device by right-clicking on the device in question in the Devices view under Configuration/Storage in the vSphere Client.  From that point you can remove the device from your storage group.  If you check out our Symmetrix/VMware TechBook you can find an example of detaching a volume - [URL] pg. 93 walks through the process.
I'm looking for recommendations for the proper way to remove a datastore from an ESXi 5 server?  When I say "remove" think of it as decommissioning the datastore; we don't need to save the data and we are not planning on bringing it back.  In the past, we would unmap the underlying volume and remove it from the storage group but this has caused a few problems.  The our current process is very slow but appears to be safe in that we don't have issues with inaccessible devices hanging the ESXi servers.  Thanks for your feedback.
What is the proper way to remove a datastore from an ESXi 5 server?