A mapped network drive is a drive/partition/folder on a drive somewhere else on the network (ie - not in the current system). Mapping simply means assigning a drive letter to that particular resource, to make it seem as if it's a drive in the current system.
So you can't delete it - as it's somewhere else on the network. All you can do is disconnect it. If the device it's on is still on the network, all you can do is make it unavailable to other systems, and you do that from the system it's actually on.