You are right - i can delete the network if it is not in use. But that is not the point - maybe i was not clear enough.
I want to delete a vdc not the network. and to delete it all vdc networks have to be removed. If it is a shared network it might be used in aother vdc as you indicated, and therefore cant't/shouldn't be deleted. So instead of deleting the vDC we have to disable it, until the network is not used any longer in other vdcs.
In the meantime we solved it by creating separate Org VDc Networks, which are connected to the same external network. Thats the most flexible solution. Since we create the network configuration with vCenter Orchestrator that task is invisible for the user creating the network.