You should be able to delete a network so long as it's not in use.
Shared or not, that doesn't matter.
However, if you do share it, then VMs in other Org vDCs could have NIC Cards assigned to it. This is either in My Cloud or in your Catalog(s).
I thought I had a DB query to show vApps/VMs assigned to a given org network, which is to show that the Network is actually in use and that the error is genuine.